American Journal of Respiratory and Critical Care Medicine

Rationale: Physical activity enhances uptake of air pollutants in the lung, possibly augmenting their harmful effects on chronic lung disease during exercise.

Objectives: To examine whether benefits of physical activity with respect to the risk of asthma and chronic obstructive pulmonary disease (COPD) are moderated by exposure to high air pollution levels in an urban setting.

Methods: A total of 53,113 subjects (50–65 yr) from the Danish Diet, Cancer, and Health cohort reported physical activity at recruitment (1993–1997) and were followed until 2013 in the National Patient Register for incident hospitalizations for asthma and COPD. Levels of nitrogen dioxide (NO2) were estimated at subject residences at the time of recruitment. We used Cox regression to associate physical activities and NO2 (high/medium/low) with asthma and COPD, and then introduced an interaction term between each physical activity and NO2.

Measurements and Main Results: A total of 1,151 subjects were hospitalized for asthma and 3,225 for COPD during 16 years. We found inverse associations of participation in sports (hazard ratio [95% confidence interval]: 0.85 [0.75–0.96]) and cycling (0.85 [0.75–0.96]) with incident asthma, and of participation in sports (0.82 [0.77–0.89]), cycling (0.81 [0.76–0.87]), gardening (0.88 [0.81–0.94]), and walking (0.85 [0.75–0.95]) with incident COPD admissions. We found positive associations between NO2 and incident asthma (1.23 [1.04–1.47]) and COPD (1.15 [1.03–1.27]) hospitalizations (comparing ≥21.0 μg/m3 to <14.3 μg/m3). We found no interaction between associations of any physical activity and NO2 on incident asthma or COPD hospitalizations.

Conclusions: Increased exposure to air pollution during exercise does not outweigh beneficial effects of physical activity on the risk of asthma and COPD.

Scientific Knowledge on the Subject

It is well known that physical activity reduces, while air pollution increases, the risk of asthma and chronic obstructive pulmonary disease (COPD). However, physical activity enhances uptake of air pollutants in the lung, possibly augmenting their harmful effects on chronic lung disease during exercise.

What This Study Adds to the Field

In this study of 53,113 elderly subjects living in two of the largest urban areas of Denmark, we found that beneficial effects of doing sports, cycling, and gardening in reducing risk of new asthma and COPD hospitalizations were not moderated in subjects residing in areas with the highest compared to those with the lowest air pollution levels. The long-term benefits of physical activity in preventing development of asthma and COPD in elderly subjects outweigh the risks associated with exposure to air pollution.

As the older adult population living in cities expands globally (1), alongside the number of people living with chronic diseases, there is a growing focus on healthier living in urban settings, where air pollution presents a major environmental stressor (2). Public health campaigns and policies promoting exercise and active transport (cycling and walking) in cities are increasingly being used as strategies in combating the pandemic of physical inactivity (3), as well as lowering traffic congestion, air pollution, and greenhouse gas emissions (4, 5). However, the major challenge is the tradeoff between the health benefits and potential harms due to amplified exposure to air pollution during outdoor exercise in cities (5, 6).

Health impact assessment studies have generally found that active transport increases scenarios that provide net health benefits, outweighing risks (6), but focus mostly on mortality, and few on morbidity, specifically respiratory morbidity, one of the major health burdens related to air pollution (2). Long-term exposure to air pollution adversely affects lung function (7) and can lead to development and exacerbation of asthma and chronic obstructive pulmonary disease (COPD) in the elderly (812). Physical activity, even at modest levels, reduces the risk of premature mortality (13), asthma, and COPD (1416), and improves the prognosis in patients with COPD and asthma (17). However, enhanced deposition of air pollutants in the lung due to higher minute ventilation during exercise (18) and cycling (19) may augment short-term harmful effects of air pollution. Reduced lung function was found in subjects with asthma after walking on busy urban streets (20, 21), in healthy subjects running near a major highway (22), cycling during rush hour on a high-traffic route (19), or hiking on high–air pollution days (23). Whether these acute adverse effects of short-duration exposures to high air pollution during exercise on lung function are transient and reversible, or accumulate and possibly outweigh long-term benefits of physical activity to the lung in the elderly, is not fully understood. Participation in sports was associated with development of asthma in children residing in areas with high ozone levels, but not in areas with low ozone levels (24), suggesting an interaction between air pollution and physical activity in children. A study in young adults exercising in high– and low–air pollution settings in Barcelona found that benefits of exercise for pulmonary function outweighed the risks related to air pollution exposure (25). We have similarly recently reported that benefits of physical activity in reducing mortality outweighed the risk related to increased exposure to air pollution exposure during physical activity, except for respiratory disease mortality, for which the benefits of cycling and gardening were somewhat reduced, but not annulled (26). However, as few people specifically die from respiratory disease (6% of total mortality), data on morbidity are needed to clarify whether patients with chronic respiratory disease are especially susceptible to air pollution during exercise.

In a large, prospective, urban cohort, we estimated the effect of leisure time and transport-related physical activity (sports, cycling, gardening, and walking) on incident and recurring asthma and COPD hospitalizations, and examined whether these were modified by long-term exposure to air pollution.

Some of the results of this study have been previously reported in the form of an abstract (27).

Design and Study Population

The study was based on the Danish Diet, Cancer, and Health cohort with 57,053 subjects who were born in Denmark, aged 50–64 years, and lived in Copenhagen or Aarhus at recruitment (1993–1997) (28). The participants completed a questionnaire on diet, smoking, alcohol consumption, education, occupation, physical activity, and medical history, and provided blood samples, blood pressure, height, and weight measurements at enrolment. Relevant Danish ethics committees and data protection agencies approved the study, and all participants provided written, informed consent.

Asthma and COPD Definition

Using unique personal identification numbers, we linked the cohort to the Danish National Patient Register dating back to 1978 (29) to identify hospital admissions using primary discharge diagnoses for asthma (International Classification of Diseases, 10th revision: J45–46 [30]) and COPD (J45–46), and to the Central Population Registry (31) to obtain date of death or emigration until December 31st, 2012. The incident (new) admission (date of first hospital admission) for asthma or COPD between cohort baseline (1993–1997) and December 31, 2012, was the main outcome in subjects who had not been admitted to hospital for asthma or COPD, respectively, before cohort baseline dating back to 1978. In addition, in subjects who had been admitted to hospital for asthma or COPD before baseline, first readmission for asthma or COPD after cohort baseline was used as the main outcome.

Physical Activity Definition

Physical activity information was collected by a self-administered, interviewer-checked questionnaire in which leisure time and utilitarian (traveling to and from work, shopping, etc.), transport-related physical activity was reported as hours per week spent on sports, cycling, gardening, walking, housework, and “do-it-yourself” activities. Information was collected separately for winter and summer of the previous year, and the two values were averaged, so that being active implies at least 30 minutes spent on a specific activity per week. The physical activity questions have been validated by findings of high correlation between self-reported physical activity estimates with the accelerometer measurements of total metabolic equivalent in 182 subjects (32) and with combined heart rate and movement sensing measurements in 1,941 subjects (33). Here, we focus on participation in sports, cycling, gardening, and walking at least 30 minutes per week, which are relevant as outdoor physical activitities pertinent to exposure to air pollution, and used earlier in the related study on mortality (26). Accounting for the amount of physical activity did not alter associations of mortality with participation in physical activities in this cohort in an earlier study (34). Thus, we focused on the effect of participation (yes/no) in sports, cycling, gardening, and walking on mortality in the main analyses, and estimated effect of the amount of time spent on each activity (<0.5 h/wk, 0.5–4 h/wk, >4 h/wk) and its interaction with air pollution as sensitivity analyses.

Air Pollution Exposure

The outdoor concentration of the traffic-related pollutant, nitrogen dioxide (NO2), was calculated at the residential addresses of cohort participants with the Danish AirGIS dispersion modeling system (see http://www.dmu.dk/en/air/models/airgis/) (35). AirGIS, described in more detail elsewhere (11, 12, 26), is based on a geographical information system, and provides estimates of traffic-related air pollution with high temporal (1-yr averages) and spatial (address-level) resolution, which have been validated, as high correlation was found between AirGIS-estimated and measured NO2 values (36). AirGIS has been used in earlier studies in this cohort, reporting association of NO2 with asthma and COPD, respectively (11, 12, 26). We used the mean of annual concentrations of NO2 at residential addresses of each cohort participant at cohort baseline address as a proxy of average exposure to traffic-related air pollution during exercise. We defined an indicator of high (upper 25th percentile of exposure range: ≥21.0 μg/m3), medium (14.3–21.0 μg/m3), and low (lower 25th percentile of exposure range: <14.3 μg/m3) exposure to NO2.

Statistical Analysis

We used Cox proportional hazards model with age as the underlying time scale to examine the associations of the participation in sports, cycling, gardening, and walking, simultaneously, with asthma (incident admissions and readmissions) and COPD (incident admissions and readmissions), respectively, in four separate models. The follow-up started on the date of recruitment into the cohort (1993–1997) and ended at the date of first asthma or COPD admission (or first readmission for those who had admission before baseline), date of death, emigration, or December 31st, 2012, whichever came first. We evaluated the effect of participation in physical activities in two steps: crude model, adjusted for age (underlying time scale), and fully adjusted model for age, sex, occupational physical activity, obesity (body mass index ≥ 30 kg/m3), smoking status, smoking intensity, smoking duration, environmental tobacco smoke, marital status, and occupational exposure, all defined as shown in Table 1. As a sensitivity analysis, we fit an additional model without obesity, which is a confounder, but also a potential mediating variable of an association between physical activity and asthma/COPD. We also evaluated the effect of time spent on each physical activity (<0.5 h/wk, 0.5–4 h/wk, >4 h/wk), as well as exposure to NO2 (low, <14.3 μg/m3; medium, 14.3–21.0 μg/m3; high, ≥21.0 μg/m3) on incident asthma and COPD admissions and readmissions. Effect modification of associations between the four physical activities (yes/no) and incident and readmissions for asthma and COPD by exposure to NO2 (low, < 14.3 μg/m3; medium, 14.3–21.0 μg/m3; high, ≥21.0 μg/m3) was evaluated by introducing an interaction term into the model, and tested using likelihood ratio tests. As a sensitivity analysis, we examined the association of time spent on each physical activity in more detailed categories (<0.5 h/wk, 0.5–2 h/wk, 2–4 h/wk, >4 h/wk), and present these in the online supplement. We also estimated the effect of participation in each physical activity on incident asthma and COPD admissions in a stratified model in each stratum of the cohort by level of NO2 (low, <14.3 μg/m3; medium, 14.3–21.0 μg/m3; high, ≥21.0 μg/m3), and present these in the online supplement. We checked the proportional hazards assumption by testing for a nonzero slope in a generalized linear regression of the scaled Schoenfeld residuals on functions of time (37). Results are presented as hazard ratios (HRs) with 95% confidence intervals, estimated with stcox in Stata 11.2.

Table 1. Characteristics of the Danish Diet, Cancer, and Health Cohort (n = 53,113) by First Asthma or Chronic Obstructive Pulmonary Disease Hospitalization Status at Follow-up for Full Cohort

 Total (n = 53,113)Asthma (n = 1,347)COPD (n = 3,476)
Asthma-free at baseline, n (%)52,572 (99.0)1,151 (2.2)
Had asthma hospitalization before baseline, n (%)541 (1.0)196 (14.5)
COPD-free at baseline, n (%)52,674 (99.2)3,225 (6.2)
Had COPD hospitalization before baseline, n (%)439 (0.8)221 (50.3)
Baseline cohort covariates   
 Mean ± SD age at cohort entry, yr56.7 ± 4.457.1 ± 4.558.1 ± 4.4
 Males, n (%)25,251 (47.5)514 (38.2)1,576 (45.3)
 Participation in sports, n (%)28,680 (54.0)665 (49.4)1,377 (39.6)
 Cycling, n (%)36,044 (67.9)851 (63.2)2,009 (57.8)
 Gardening, n (%)38,883 (73.2)915 (68.0)2,231 (64.2)
 Walking, n (%)49,332 (92.9)1,241 (92.1)3,127 (90.0)
 Mean (SD) time/wk participating in sports*, h2.4 (2.3)2.4 (2.4)2.5 (2.5)
 Mean (SD) time/wk cycling*, h3.2 (3.4)3.4 (3.3)3.4 (3.5)
 Mean (SD) time/wk gardening*, h3.0 (3.2)3.1 (3.2)3.7 (3.9)
 Mean (SD) time/wk walking*, h4.4 (4.7)4.5 (4.6)5.1 (5.5)
 Never smoked, n (%)18,827 (35.5)453 (33.6)245 (7.1)
 Previously smoked, n (%)15,224 (28.7)424 (31.5)652 (18.8)
 Currently smoked, n (%)19,062 (35.9)470 (34.9)2,579 (74.2)
 Mean (SD) smoking duration, yr19.0 (17.2)19.8 (17.1)34.5 (12.8)
 Mean (SD) smoking intensity, g/d6.3 (10.4)5.5 (9.3)14.1 (11.9)
 Environmental tobacco smoke, n (%)34,109 (64.2)883 (65.6)3,040 (87.5)
 Occupational exposure, n (%)14,884 (28.0)369 (27.4)1,284 (36.9)
 Obesity (BMI ≥ 30 kg/m2), n (%)7,709 (14.5)237 (17.6)546 (15.7)
 Sedentary work, n (%)19,040 (35.9)454 (33.7)895 (25.8)
 Standing work, n (%)9,163 (17.3)220 (16.3)508 (14.6)
 Manual work, n (%)10,756 (20.3)243 (18.0)607 (17.5)
 Heavy manual work, n (%)2,459 (4.6)60 (4.5)197 (5.7)
 Unemployed, n (%)11,696 (22.0)370 (27.5)1,269 (36.5)
 <8 yr of education, n (%)17,534 (33.0)472 (35.0)1,662 (47.8)
 8–10 yr of education, n (%)24,529 (46.2)622 (46.2)1,381 (39.7)
 ≥10 yr of education, n (%)11,050 (20.8)253 (18.8)434 (12.5)
 Single, n (%)3,126 (5.9)69 (5.1)192 (5.5)
 Married, n (%)38,197 (71.9)916 (68.0)2,184 (62.8)
 Divorced, n (%)8,871 (16.7)280 (20.8)835 (24.0)
 Widow/widower, n (%)2,919 (5.5)82 (6.1)265 (7.6)
Air pollution levels at baseline year (1993–1997)   
 Mean ± SD NO218.8 ± 6.619.3 ± 6.919.6 ± 7.1
 Low 25th percentile (<14.3 μg/m3)13,015 (24.9)273 (20.8)683 (20.1)
 Medium (14.3–21.0 μg/m3)25,822 (49.4)664 (50.5)1,691 (49.7)
 High (>21.0 μg/m3)13,425 (25.7)377 (28.7)1,029 (30.2)

Definition of abbreviations: BMI = body mass index; COPD = chronic obstructive pulmonary disease.

Note: the arrangement of the entries in the baseline cohort variate column is based on Reference 26.

*Duration of participation in physical activities for participants.

Indicator of exposure to smoke in the home and/or at work for at least 4 h/d.

Defined as at least 1 year of employment (and reflects occupations related to chronic lung disease) in: mining; electroplating; shoe or leather manufacture; welding; painting; steel mill; shipyard; construction (roof, asphalt, or demolition); truck, bus, or taxi driver; asbestos or cement manufacture; asbestos insulation; glass, china, or pottery manufacture; butcher; auto mechanic; waiter; or cook.

Of the 57,053 cohort members, 571 were excluded due to cancer diagnosis before baseline, 2,210 due to missing air pollution exposure assessment or residential address at recruitment, and 1,159 due to missing information on a potential confounder. Of the remaining 53,113 people, 1,347 were admitted for asthma (848,103 person-years) and 3,476 for COPD (836,720 person-years) during follow-up of a mean of 16 years between baseline (1993–1997) and 2013. Of the 52,572 people without asthma admissions before baseline, 1,151 had incident admissions (841,771 person-years), whereas, of the 541 people hospitalized for asthma before baseline, 196 were readmitted for asthma (6,242 person-years). Of the 52,674 people without COPD admissions before baseline, 3,255 had incident admissions (841,771 person-years), whereas, of the 439 people hospitalized for COPD before baseline, 221 were readmitted for COPD (6,242 person-years).

Mean age at baseline was 56.7 years and 47.5% of the study subjects were men (Table 1). The majority of the study subjects were physically active: 54.0% participated in sports; 67.9% cycled; 73.2% gardened; and 92.9% walked. Study subjects who participated in activities spent, on average, 2.4 h/wk participating in sports, 2.2 h/wk cycling, 2.2 h/wk gardening, and 4.1 h/wk walking (Figure 1). Cohort participants who developed asthma or COPD during follow-up generally were less physically active at baseline compared with the total cohort. The mean concentration of NO2 at residence was 18.8 μg/m3 in the total cohort (Figure 2), and 19.3 μg/m3 and 19.6 μg/m3 in patients with asthma and COPD, respectively.

We found statistically significant inverse associations between participation in sports (hazard ratio [95% confidence interval]: 0.85 [0.75–0.96]) and cycling (0.85 [0.75–0.96]) and incident asthma in the fully adjusted model, and no associations with gardening or walking, or any activity and asthma readmissions in participants with asthma (Table 2). We found statistically significant inverse associations between participation in all activities and incident COPD admissions, stronger than those observed for asthma: sports (0.82 [0.77–0.89]); cycling (0.81 [0.76–0.87]); gardening (0.88 [0.81–0.94]); and walking (0.85 [0.75–0.95]). Associations of all physical activities with COPD readmissions were not statistically significant due to small numbers, but of similar magnitude to those for incident COPD. Estimates of association between physical activities and asthma and COPD (both incident and readmissions) remained unchanged or were only slightly attenuated in a model without obesity (data not shown).

Table 2. Association between Hospital Admission for Asthma or Chronic Obstructive Pulmonary Disease and Participation (Yes/No) in Physical Activity in 53,113 Participants in the Diet, Cancer, and Health Cohort

Physical ActivityCrude* Model [HR (95% CI)]Fully Adjusted Model [HR (95% CI)]
Asthma  
 Incident asthma admissions (n = 1,151) in 52,572 asthma-free subjects at baseline  
  Sports0.85 (0.75–0.95)0.85 (0.75–0.96)
  Cycling0.85 (0.75–0.96)0.85 (0.75–0.96)
  Gardening0.82 (0.72–0.93)0.89 (0.78–1.01)
  Walking0.90 (0.73–1.12)0.88 (0.71–1.09)
 Asthma readmissions (n = 196) in 541 subjects with asthma before baseline  
  Sports0.97 (0.72–1.29)0.93 (0.69–1.26)
  Cycling0.96 (0.71–1.29)0.98 (0.72–1.32)
  Gardening0.80 (0.60–1.08)0.86 (0.63–1.18)
  Walking1.18 (0.65–2.13)1.18 (0.64–2.18)
COPD  
 Incident COPD admissions (n = 3,255) in 52,674 COPD-free subjects at baseline  
  Sports0.60 (0.55–0.64)0.82 (0.77–0.89)
  Cycling0.73 (0.68–0.78)0.81 (0.76–0.87)
  Gardening0.72 (0.67–0.78)0.88 (0.81–0.94)
  Walking0.79 (0.70–0.88)0.85 (0.75–0.95)
 COPD readmissions (n = 221) in 439 subjects with COPD before baseline  
  Sports0.85 (0.63–1.16)0.87 (0.63–1.19)
  Cycling0.80 (0.61–1.06)0.85 (0.65–1.12)
  Gardening0.64 (0.49–0.83)0.77 (0.58–1.01)
  Walking0.77 (0.52–1.13)0.81 (0.55–1.20)

Definition of abbreviations: CI = confidence interval; COPD = chronic obstructive pulmonary disease; HR = hazard ratio.

*Adjusted mutually for the other three physical activities.

Adjusted for sex, smoking status, obesity, environmental tobacco smoke, occupational smoke, education, physical activity at work, and mutually for the other three physical activities.

There was generally no additional benefit of increasing time spent on physical activity for incident asthma and COPD admissions, where the biggest benefit was observed with moderate activity between 0.5 and 4 h/wk (Table 3 and Table E1 in the online supplement). However, some indication was observed of increased benefit of spending greater than 4 h/wk as compared with spending 0.5–4 h/wk cycling and gardening in reducing risk of readmission for asthma, as well in participating in sports and gardening for readmission for COPD, although none was statistically significant, and these results need to be considered with caution, due to the small numbers.

Table 3. Association* between Hospital Admission for Asthma or Chronic Obstructive Pulmonary Disease and Time Spent in Physical Activity in 53,113 Participants in the Diet, Cancer, and Health Cohort

 Sport [HR (95% CI)]Cycling [HR (95% CI)]Gardening [HR (95% CI)]Walking [HR (95% CI)]
Asthma-free at baseline (52,572), 1,151 (2.2%) incident asthma admissions    
 <0.5 h/wk1.01.01.01.0
 0.5–4 h/wk0.83 (0.74–0.95)0.81 (0.71–0.93)0.87 (0.76–1.00)0.88 (0.71–1.10)
 >4 h/wk0.98 (0.78–1.22)0.93 (0.79–1.10)0.92 (0.77–1.10)0.83 (0.66–1.05)
Asthma admission before baseline (541), 196 (36.2%) asthma readmissions    
 <0.5 h/wk1.01.01.01.0
 0.5–4 h/wk0.95 (0.69–1.30)1.10 (0.80–1.51)0.96 (0.68–1.34)1.18 (0.62–2.26)
 >4 h/wk1.22 (0.71–2.09)0.78 (0.49–1.25)0.90 (0.58–1.39)1.14 (0.59–2.19)
COPD-free at baseline (52,674), 3,255 (6.2%) incident COPD admissions    
 <0.5 h/wk1.01.01.01.0
 0.5–4 h/wk0.82 (0.76–0.89)0.81 (0.75–0.87)0.85 (0.78–0.92)0.82 (0.73–0.93)
 >4 h/wk0.90 (0.78–1.04)0.84 (0.76–0.94)0.95 (0.86–1.05)0.87 (0.77–0.99)
COPD admission before baseline (439), 221 (50.3%) COPD readmissions    
 <0.5 h/wk1.01.01.01.0
 0.5–4 h/wk0.89 (0.64–1.24)0.85 (0.63–1.16)0.81 (0.59–1.10)0.73 (0.48–1.11)
 >4 h/wk0.74 (0.34–1.60)0.84 (0.55–1.27)0.74 (0.51–1.08)0.85 (0.55–1.31)

For definition of abbreviations, see Table 2.

*Adjusted for sex, smoking status, obesity, environmental tobacco smoke, occupational smoke, education, physical activity at work, and mutually for the other three physical activities.

We found statistically significant, positive exposure–response relationships between exposure to NO2 at residence and incident asthma (1.23 [1.04–1.47], comparing high [≥21.0 μg/m3] to low [<14.3 μg/m3] NO2 levels) and even stronger associations with readmission for asthma (1.43 [0.91–2.24]) (Table 4). Similarly, we also detected significant, positive exposure–response relationships with incident COPD admissions (1.15 [1.03–1.27]) and considerably enhanced associations with readmissions for COPD (1.52 [0.99–2.32], comparing high [≥21.0 μg/m3] to low [<14.3 μg/m3] NO2 levels) (Table 4). Data on incident asthma and COPD in relation to residential NO2 with follow-up until 2006 in this cohort have previously been published (11, 12).

Table 4. Association of Asthma and Chronic Obstructive Pulmonary Disease with Annual Mean Level of Nitrogen Dioxide at Cohort Baseline (1993–1997) among 53,125 Participants in Diet, Cancer, and Health Cohort

 Crude Model [HR (95% CI)]Fully Adjusted* Model [HR (95% CI)]
Asthma-free at baseline (52,572), 1,151 (2.2%) incident asthma admissions  
 Low NO2 (<14.3 μg/m3)1.01.0
 Medium NO2 (14.3–21.0 μg/m3)1.20 (1.03–1.40)1.16 (0.99–1.35)
 High NO2 (≥21.0 μg/m3)1.32 (1.12–1.57)1.23 (1.04–1.47)
Asthma admission before baseline (541), 196 (36.2%) asthma readmissions  
 Low NO2 (<14.3 μg/m3)1.01.0
 Medium NO2 (14.3–21.0 μg/m3)1.26 (0.86–1.83)1.12 (0.75–1.65)
 High NO2 (≥21.0 μg/m3)1.62 (1.05–2.48)1.43 (0.91–2.24)
COPD-free at baseline (52,674), 3,255 (6.2%) incident COPD admissions  
 Low NO2 (<14.3 μg/m3)1.01.0
 Medium NO2 (14.3–21.0 μg/m3)1.27 (1.16–1.40)1.18 (1.08–1.30)
 High NO2 (≥21.0 μg/m3)1.46 (1.33–1.62)1.15 (1.03–1.27)
COPD admission before baseline (439), 221 (50.3%) COPD readmissions  
 Low NO2 (<14.3 μg/m3)1.01.0
 Medium NO2 (14.3–21.0 μg/m3)1.37 (0.97–1.94)1.36 (0.93–1.98)
 High NO2 (≥21.0 μg/m3)1.57 (1.07–2.32)1.52 (0.99–2.32)

For definition of abbreviations, see Table 2.

*Adjusted for sex, smoking status, obesity, environmental tobacco smoke, occupational smoke, education, physical activity at work, and mutually for the other three physical activities.

There was no statistically significant effect modification of the inverse associations between any of the four physical activities and asthma (Table 5) and COPD (Table 6) incident admissions or readmissions by the level of exposure to air pollution assessed as NO2 at the residential address. However, for some forms of physical activity, the risk associated with air pollution exposure was rather similar for both active and inactive subjects with regard to incident asthma (gardening) and COPD admissions (sports, gardening, and walking), whereas, for cycling participants, risk estimates related to exposure were lower for all groups. Similarly, we found inverse associations of participation in sports, cycling, gardening, and walking with incident asthma and COPD admissions for subjects living in areas with low, medium, or high levels of NO2 (see Table E2). For example, for cycling, we detected inverse associations with incident asthma admissions in areas with low (0.84 [0.64–1.10]), medium (0.88 [0.73–1.05]), and high (0.74 [0.58–0.93]) NO2 levels. Similarly for incident COPD admissions, we found statistically significant inverse association with cycling in all areas (low NO2, 0.79 [0.67–0.93]; medium NO2, 0.83 [0.74–0.92]; and high NO2, 0.81 [0.71–0.93]).

Table 5. Effect Modification of Association* between Hospital Admissions for Asthma with Participation in Physical Activities by Nitrogen Dioxide Level in the Diet, Cancer, and Health Cohort

 Low NO2 (<14.3 μg/m3) [HR (95% CI)]Medium NO2 (14.3–21.0 μg/m3) [HR (95% CI)]High NO2 (≥21.0 μg/m3) [HR (95% CI)]P Value for Interaction
Incident asthma admissions (n = 1,151) in 52,572 asthma-free subjects at baseline    
 Sports   0.61
  No1.001.46 (1.16–1.84)1.48 (1.16–1.90) 
  Yes0.91 (0.69–1.18)0.89 (0.66–1.22)1.01 (0.72–1.42) 
 Cycling   0.46
  No1.001.25 (0.98–1.60)1.48 (1.13–1.94) 
  Yes0.77 (0.59–1.00)1.17 (0.85–1.61)1.01 (0.71–1.42) 
 Gardening   0.39
  No1.001.11 (0.79–1.56)1.24 (0.88–1.74) 
  Yes0.76 (0.55–1.07)1.31 (0.89–1.91)1.25 (0.84–1.85) 
 Walking   0.79
  No1.001.63 (0.94–2.83)1.77 (0.96–3.25) 
  Yes1.02 (0.62–1.67)0.83 (0.47–1.48)0.83 (0.44–1.55) 
Asthma readmissions (n = 196) in 541 subjects with asthma admission before baseline    
 Sports   0.50
  No1.001.32 (0.76–2.29)1.38 (0.74–2.58) 
  Yes0.97 (0.50–1.87)0.84 (0.39–1.79)1.25 (0.54–2.90) 
 Cycling   0.53
  No1.001.51 (0.80–2.85)2.04 (1.03–1.54) 
  Yes1.34 (0.67–2.68)0.70 (0.32 –1.54)0.61 (0.25–1.48) 
 Gardening   0.99
  No1.001.19 (0.53–2.64)1.50 (0.68–3.30) 
  Yes0.91 (0.40–2.04)1.03 (0.41–2.60)1.03 (0.39–2.72) 
 Walking   0.62
  No1.000.61 (0.13–2.80)0.77 (0.17–3.40) 
  Yes0.68 (0.20–2.30)2.06 (0.43–9.87)2.10 (0.45–9.85) 

Definition of abbreviations: CI = confidence interval; HR = hazard ratio.

*Adjusted for sex, smoking status, obesity, environmental tobacco smoke, occupational smoke, education, physical activity at work, and mutually for the other three physical activities.

Table 6. Effect Modification of Association* between Hospital Admissions for Chronic Obstructive Pulmonary Disease with Participation in Physical Activities by Nitrogen Dioxide Level in the Diet, Cancer, and Health Cohort

 Low NO2 (<14.3 μg/m3) [HR (95% CI)]Medium NO2 (14.3–21.0 μg/m3) [HR (95% CI)]High NO2 (≥21.0 μg/m3) [HR (95% CI)]P Value for Interaction
Incident COPD admissions (n = 3,255) in 52,674 COPD-free subjects at baseline    
 Sports   0.11
  No1.001.16 (1.02–1.31)1.17 (1.03–1.34) 
  Yes0.75 (0.64–0.88)1.09 (0.91–1.32)1.23 (1.01–1.51) 
 Cycling   0.89
  No1.001.17 (1.02–1.35)1.27 (1.09–1.48) 
  Yes0.78 (0.67–0.92)1.04 (0.87–1.26)1.02 (0.83–1.24) 
 Gardening   0.20
  No1.001.02 (0.84–1.25)1.12 (0.92–1.36) 
  Yes0.78 (0.64–0.94)1.23 (0.98–1.53)1.18 (0.94–1.49) 
 Walking   0.57
  No1.001.04 (0.79–1.38)1.14 (0.84–1.55) 
  Yes0.75 (0.59–0.95)1.17 (0.87–1.57)1.13 (0.82–1.56) 
COPD readmissions (n = 221) in 439 subjects with COPD before baseline    
 Sports   0.57
  No1.001.58 (0.99–2.53)2.02 (1.22–3.35) 
  Yes1.21 (0.65–2.26)0.71 (0.34–1.49)0.65 (0.27–1.54) 
 Cycling   0.40
  No1.001.68 (1.05–2.67)1.94 (1.13–3.32) 
  Yes1.13 (0.60–2.14)0.59 (0.28–1.25)0.70 (0.31–1.58) 
 Gardening   0.33
  No1.002.01 (1.03–3.93)2.53 (1.28–5.01) 
  Yes1.31 (0.65–2.64)0.58 (0.26–1.29)0.56 (0.23–1.33) 
 Walking   0.86
  No1.001.54 (0.59–3.99)1.53 (0.50–4.70) 
  Yes0.81 (0.34–1.96)0.89 (0.32–2.46)1.16 (0.35–3.80) 

For definition of abbreviations, see Table 2.

*Adjusted for sex, smoking status, obesity, environmental tobacco smoke, occupational smoke, education, physical activity at work, and mutually for the other three physical activities.

We found that leisure time participation in sports, cycling (leisure and utilitarian), gardening, and walking were associated with lower risk, whereas air pollution at residence was associated with increased risk of asthma and COPD hospitalizations. We found no effect modification of association between any physical activity and incident asthma or COPD hospitalizations by NO2. Results were similar for the risk of readmission for asthma or COPD in patients with pre-existing disease. The level of traffic-related air pollution did not significantly modify beneficial effects of physical activity on asthma or COPD development and exacerbation.

Our finding of statistically significant inverse association between physical activity and asthma and COPD incidence, defined objectively as incident hospital admission, corroborates existing evidence (1416). We found a 15% lower rate of incident asthma admissions in the subjects participating in sports and cycling, in agreement with recent meta-analyses by Eijkemans and colleagues (16), who reported a 12–13% lower incidence of asthma in subjects with high physical activity. We found no exposure–response relationship between exercise and incident asthma admission (Table 2), with moderate activity (0.5–4 h/wk) being most beneficial in reducing risk of new asthma hospitalization, and no additional benefit of participating in physical activity over 4 h/wk. These findings of moderate activity being most beneficial for asthma are in line with the study by Del Giacco and colleagues (38), who showed that both inactivity and highly rigorous physical activity may induce asthma symptoms, later known as exercise-induced asthma or exercise-induced bronchoconstriction. We did not detect statistically significant signs of benefit of physical activity with respect to asthma readmissions (Table 2), in contrast to a study finding lower risk of asthma exacerbation with increasing physical activity (39).

The subjects participating in sports, cycling, gardening, and walking had 18, 19, 12, and 15%, respectively, lower rates of incident COPD admissions than nonparticipants. These estimates add new evidence to the limited literature on the effect of physical activity on COPD incidence (14, 15, 40, 41). Our results are comparable to those from Garcia-Aymerich and colleagues (15), who found a 24% lower risk of COPD development in subjects with moderate and high physical activity compared with those with low physical activity. Furthermore, the strongest benefits with respect to COPD incidence were observed with moderately active subjects (0.5–4 h/wk) as compared with inactive subjects (<0.5 h/wk), and no additional benefit was seen in spending more than 4 h/wk in activity (Table 3 and Table E1). Subjects participating in sports and cycling had 13 and 15% lower rates of readmission for COPD, respectively, whereas gardening and walking also seemed to reduce readmissions for COPD by 23 and 19%, although this reduction was not statistically significant. There was no additional benefit of cycling or walking more than 4 h/wk, whereas, for sports and gardening, 26% of reduction in COPD readmission was observed for those spending more than 4 h/wk on these activities (Table 3). Our effect estimates are somewhat weaker than those from a study reporting 26–32% reduction in COPD readmissions in subjects in high physical activity as compared with moderate/low physical activity (15). Our results corroborate existing evidence that physical activity can reduce readmissions for COPD, even at moderate levels, at about 2 h/wk (15, 39, 40, 42). Thus, our results support that physical activity, even at low and moderate levels, could be effective in the prevention of development and progression of asthma and COPD among the elderly.

Adverse effects of chronic exposure to air pollution on the lung are well established (79, 11, 12). In this cohort, we have previously linked long-term exposure to NO2 to COPD incidence (11) and asthma incidence and readmissions (12), with the same definitions of asthma, COPD, and NO2, but with follow-up until 2006. Similarly, with extended follow-up until 2013 in the current study we detected statistically significant, positive, exposure–response associations between exposure to NO2 and development and exacerbation of asthma and COPD, which was strongest for readmission in existing patients with asthma and COPD (Table 4). Moreover, air pollution can be perceived as a barrier to physical activity, and several studies demonstrated lower levels of outdoor physical activity on days with poor air quality among patients with respiratory disease (43, 44).

Our results, showing that long-term benefits of physical activity with respect to asthma and COPD morbidity in elderly subjects were not statistically significantly moderated by exposure to higher levels of NO2, are novel. In particular, the benefit of cycling, with respect to risk of the asthma and COPD outcomes, appeared robust, despite air pollution exposure. Our results complement recent results by Kubesch and colleagues (25), who, in an experimental study of the interaction between short-term exposure to air pollution and physical activity, found that, after cycling in areas with high pollution, lung function improvements were not different from effects of cycling on lung function in low-pollution areas of Barcelona. Thus, Kubesch and colleagues (25), in line with our results, concluded that physical activity has beneficial effects on pulmonary function, even performed in a highly polluted environment in Barcelona, with considerably higher air pollution levels than in Copenhagen. This implies that acute damage and reductions in lung function during short, varying exercise in areas with high air pollution reported earlier (1824, 26) seem to be transient and reversible, and do not abate short- and long-term benefits of physical activity with respect to asthma and COPD. Our findings may also be explained by the relatively short duration of the physical activities in this cohort of 2–4 h/wk (Table 1), implying that additional inhaled doses of air pollutants during physical activity, which is a function of increased inhalation and duration, is only a small fraction of total inhaled dose of air pollution (45) and, therefore, not sufficient to increase the risk of chronic lung disease. Our results do, however, differ from those from McConnell and colleagues (24), who showed asthma development only in children living in areas with high ozone concentrations, and not in those living in areas of Los Angeles, California, with low ozone. The study by McConnell and colleagues (24) is not, however, directly comparable to ours in elderly subjects, as children may be more susceptible to air pollution. Furthermore, McConnell and colleagues (24) used physician-diagnosed asthma, as opposed to hospitalizations used in our study. Finally, levels of ozone and other air pollutants in Los Angeles are generally higher than those in Copenhagen (24).

The main strength of our study includes the use of the objective definition of asthma and COPD morbidity as the hospital admission in the Danish National Patient Register, a nationwide register of routinely collected data on all hospitalizations in Denmark (in- and outpatient, emergency room), with no loss to follow-up. Both asthma (46) and COPD diagnoses (47, 48) have been previously validated, showing good agreement between hospital discharge diagnoses with information from medical records on symptoms and lung function measurements. We also benefited from the large prospective cohort with well defined and validated data on confounders and physical activity (32, 33), which has been recently used in related work on physical activity and mortality in this cohort (26). We also benefited from the availability of air pollution data (35), which have been previously linked to risk of asthma (12) and COPD (11) in this cohort. Information or recall bias and differential bias in relation to exposure is minimal, as COPD and asthma incidence, vital status, and information on address used in modeling air pollution are obtained objectively from reliable population registers. Another strength of this study is the setting in Denmark, where prevalence of cycling is high, at 68%, and includes both leisure and utilitarian cycling, thus facilitating the first study of the effect of cycling on asthma and COPD.

The main weakness of the study is the use of NO2 levels at baseline residence as a proxy of air pollution level exposure during different physical activities. This proxy is reasonable for gardening, which is assumed to take place at the baseline residence, precisely where NO2 levels are modeled, but less reasonable for walking and cycling, and the lack of exact location on these activities could alter the study’s results. However, considering the age (50–65 yr) of the cohort participants, many of whom retired before cohort baseline or during follow-up, we can assume that walking and cycling occurred in close proximity to their residences. The cycling levels were comparable for subjects residing in areas with low (65%), moderate (70%), and high (68%) air pollution levels. In contrast, gardening was more common in subjects living in areas with low air pollution (85%) than in those in areas with moderate (76%) and high (57%) air pollution, likely explained by higher rates of home ownership in the suburbs than in the inner city, where pollution is highest and where it is more common to live in apartment. Thus, a very strong positive association (2.53 [1.28–5.01]) observed between NO2 and COPD readmission in nongardeners (Table 6) may be explained by higher air pollution levels in nongardeners than in gardeners. However, this observation could also be ascribed to chance, due to the relatively small number of subjects in both asthma (n = 541) and COPD (n = 439) readmission analyses. A further weakness is the nonspecific nature of the variable participation in sports, and whether it pertains to indoor or outdoor activity, as well as lack of data on physical activities after baseline. We had a relatively short duration of physical activity in this cohort, with a mean of 2–4 h/wk (Table 1). Another weakness of the study is the lack of other pollutants, including particulate matter, and, in particular, ozone, which was previously found to be a relevant modifier of an association of physical activity with asthma in children (24).

Although NO2 is a good proxy of traffic-related air pollution, the results of this study for NO2 cannot be generalized to other pollutants or ozone, and more data with particulate matter and ozone are needed. We cannot exclude the possibility that lack of interaction between air pollution and participation in sports may be due to exposure misclassification; however, data for both physical activity and air pollution have been validated, and we have detected significant associations in the expected direction with both exposures with asthma and COPD, arguing against chance findings. We lacked data on lung function and spirometry measurements, and cannot distinguish patients with asthma and COPD by disease severity. Here, we cannot determine whether exercise benefits applied to all patients across different severity levels, or possibly only those with less severe disease; thus, we cannot exclude some degree of potential reverse causality, meaning that cohort participants with unrecognized asthma or COPD at baseline had already reduced physical activity due to their disease. In some analyses, especially on readmissions for asthma and COPD, numbers were low and statistical power poor, so results for these analyses should be considered with caution. Another weakness is that the Diet, Cancer, and Health cohort participants are likely healthier than the general Danish population, implying some healthy worker effect, as it was shown that they are better educated and had higher income than nonparticipants (28). Although pollution levels in Copenhagen and Aarhus are relatively low, there are still violations of European Union limit values for NO2 of 40 μg/m3 (annual mean) at residences close to roads with heavy traffic (Figure 1). Finally, findings of this study do not necessarily pertain to sites with considerably higher air pollution levels, where new studies are needed.

Our results strengthen and complement a growing number of health impact assessment studies, which evaluate the net effects of an increase in cycling at the population level, typically as a shift from car use, and conclude that health benefits due to increased physical activity levels generally outweigh the risks related to increases in inhaled air pollution doses during cycling (6, 45, 4951).

We found that beneficial effects of physical activity and adverse effects of long-term exposure to NO2 on incident asthma and COPD hospitalizations are independent of each other, and that the benefits of physical activity are not reduced for those living in areas with high air pollution levels. Thus, we can conclude that the long-term benefits of physical activity in preventing the development of asthma and COPD in healthy, elderly subjects, and possibly as effective disease control in patients with COPD, can outweigh the risks associated with enhanced exposure to air pollution during physical activity.

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Correspondence and requests for reprints should be addressed to Zorana J. Andersen, Ph.D., Center for Epidemiology and Screening, Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, Copenhagen K 1014, Denmark. E-mail:

Author Contributions: Conception and design—Z.J.A., S.L., and M.J.N.; preparation of data for analyses and main advisor for J.E.F.—Z.J.A.; statistical analyses and interpretation—J.E.F.; drafting the manuscript for important intellectual content—Z.J.A., S.L., M.J.N., C.S.U., O.R.-N., O.H., A.T., and K.O.; all authors critically reviewed and accepted the final version of the manuscript.

This article has an online supplement, which is accessible from this issue’s table of contents at www.atsjournals.org

Originally Published in Press as DOI: 10.1164/rccm.201510-2036OC on September 21, 2016

Author disclosures are available with the text of this article at www.atsjournals.org.

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