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Obstructive lung diseases are a leading cause of morbidity and mortality in the United States. Previous analyses of OLD mortality have used only the underlying cause of death data (2, 10). Over the study period we determined that the rate of OLD-related mortality increased much more (47.3%) than the increase in the proportion of these deaths listing OLD as the UCD (6.0%), indicating the mortality increase is real. However, we could not determine how important a role OLDs played in the death of people who did not have any of these diseases listed as the UCD, with possibilities ranging from an incidental role to a major factor in the death. Over the study period, though, OLDs were listed as the UCD in only 43.3% of the death certificates that listed any OLD. National estimates of the effects of smoking, such as the smoking-attributable mortality, morbidity, and economic costs (SAMMEC) are based solely on UCD data (11). By depending exclusively on UCD data, however, these estimates may fail to account for many decedents in whom obstructive lung disease was an important factor in the death even though it was not the UCD. Although some of these missed deaths may have had a UCD that SAMMEC would classify as tobacco-related (lung cancer or cardiac disease), programs that rely entirely on UCD data may still underestimate the true effects of smoking on the population.

Researchers from Arizona examined death certificates for 577 decedents who were part of a 20-yr longitudinal study of 3,206 people (12). Of 43 decedents with antemortem evidence of severe airway obstruction (FEV₁ less than 50% of predicted and a clinical diagnosis of obstructive lung disease), only 10 (23%) had OLD listed as the underlying cause of death, whereas an additional 23 (55%) had OLD listed as a cause of death but not the UCD. Of the 184 decedents with antemortem evidence of mild to moderate OLD (either clinical diagnosis of OLD or FEV₁ from 50% to 70% of predicted), five (2%) had OLD listed as the UCD, whereas 35 (18%) had OLD listed as cause of death but not the UCD. Of the 350 decedents with no antemortem diagnosis of OLD, none had OLD listed as the UCD, and only five (1%) had OLD listed as a cause of death but not the UCD. If results of that study are an accurate reflection of what happens nationally, then most decedents with an OLD diagnosis on their death certificate have OLD, although many people with OLD may not have this diagnosis listed on the death certificate.

Our analyses have shown that OLD related mortality rates vary by gender, race, and age. Although we presented only data for any mention of OLDs, the relative rates of the different demographic groups were similar to those we obtained when we evaluated OLDs as the UCD. Overall OLD-related mortality rates among males have plateaued since the mid-1980s, although they are still increasing among men older than 85, whereas these rates among females continue to increase dramatically. These different patterns are probably related both to historical trends of current and former smoking rates and to historical trends in patterns of occupational exposures among men and women (13, 14). Women may also have a different susceptibility to the effects of continued cigarette smoking or smoking cessation than men (15, 16). When we stratified decedents by race and gender, white males had the highest age-adjusted mortality rates, followed by black males, males of other races, white females, black females and females of other races. The pattern is similar to the ever smoking rates among these various groups (13). We have, in a similar analysis, examined lung cancer mortality rates and found that black males have the highest rates, mirroring their higher rates of current smoking (17). The differences between the relative ranking of various race- and gender-specific OLD mortality rates and various race- and gender-specific lung cancer mortality rates may be related to long-term former smokers being more likely to develop OLD than lung cancer (18). We found that OLD-related mortality rates increased, both among men and among women, with increasing age (Figures 3 and 4). These findings support the observation that OLDs are diseases of aging and increase as the population ages.

Although the majority of decedents with OLD had emphysema or COPD listed, more than 136,000 had a code for asthma listed. Over 90% of these asthma-related deaths occurred in people over the age of 34, although a higher percentage of decedents with OLD who were younger than age 35 had asthma listed as being present. Trends in asthma mortality have been studied both in the United States (10) and around the world (19). Most researchers have restricted their analysis of these deaths to decedents from age 5 through 34, because of uncertainty in diagnoses of people younger than 5 and older than 34 (20). Even among decedents age 5 through 34, though, some misdiagnosis may occur (21). We found that among decedents age 1 through 34 yr, OLDs, asthma in particular, were more likely to be the UCD than among decedents in other age groups (Figure 1). Although this reporting may be accurate, it may also represent an increased tendency for clinicians to attribute a younger person's death to asthma among decedents with asthma in these age groups compared with decedents with asthma in other age groups. Even if this tendency to attribute exists, however, asthma is still under reported as an associated condition on death certificates: we would expect 6 to 7% of decedents in the 15 through 34 age group to have asthma (if every person with a diagnosis of asthma had it listed on their death certificate, even if asthma had nothing to do with the death) (22) yet only 0.7% had this diagnosis listed. Another explanation for asthma being more likely to be listed as the UCD in younger decedents might be the presence of fewer comorbid conditions among children and young adults.

An advantage of using the Multiple-Cause Mortality Files is that we can search for comorbid factors. OLDs are thought to be associated with lung cancer (23), and we found that 9.9% of decedents with OLD also had lung cancer listed. Although only 0.8% of all decedents with OLD had pneumoconiosis, we found that 34.3% of decedents with a diagnosis of pneumoconiosis also had an OLD, confirming that OLD is a common complication of pneumoconiosis (14).

Tobacco use is the most important risk factor for the development of OLD (6). Tobacco use disorder (ICD-9 305.1), however, was listed on only 2.8% of the death records that listed OLD. Although this proportion increased from 0.5% in 1979 to 5.1% in 1993, tobacco-use disorder remains under reported. This underreporting of tobacco use disorder is similar to that reported by researchers in Wisconsin, who found that this code (ICD-9 305.1) was listed on only 93 (8.4%) of 1,110 death certificates in which OLD was the UCD (24). This finding can be contrasted with results from a study from Oregon, where physicians have to answer the question on the death certificate “Did tobacco use contribute to the death?” In that study, physicians responded that in 1,919 (76.2%) of 2,520 OLD deaths, tobacco use definitely or probably contributed to the death (25).

Using death certificate data for epidemiologic studies has certain limitations. Death certificate data are not independently validated, and some degree of misclassification inevitably occurs. Death certificates also have no data on the severity of chronic diseases such as OLDs, and data on risk factors such as occupational exposures, family history, and tobacco use are not universally collected.

OLDs are an important cause of mortality in the United States, yet their importance may be underestimated because they are frequently not listed as the UCD. OLD-related mortality rates have stabilized among males overall, although they continue to increase among males age 85 and older and among females. Sustained public health efforts in further reducing smoking and reducing occupational and environmental exposures, along with appropriate intervention in people with early stages of OLD (26) may help reduce the number of people who die from this group of diseases.