Global warming and related climate changes affect respiratory health directly through temperature, humidity, and extreme climate events (such as storms, floods, wildfires, and droughts), or indirectly by increasing air pollution, pollens, and molds, and by changing the habitat, thus promoting vectors for transmission of infectious diseases (4). Climate change may particularly impact migrants and refugees, due to coexisting exposure to environmental hazards (5) or inadequate access to health care (6). Forced displacement of people is associated with acute respiratory infections and tuberculosis (5), and migration from rural to urban areas has been linked to allergic diseases such as asthma (7). Over the coming decades, the number of people escaping the consequences of climate change (desertification, famine, flooding, extreme weather, and natural disasters) will likely increase. Thus, research on this public health threat is needed (7).

Air pollution is a ubiquitous risk factor for morbidity and mortality from respiratory diseases (8). Global estimates of the population-attributable fractions of disability-adjusted life-years (DALYs) due to total occupational risks, and indoor and ambient air pollutants, are as follows: asthma, 44%; chronic obstructive pulmonary disease (COPD), 35%; lung cancer, 17–45%; and acute lower respiratory infections, 35% (9, 10). More than 4,241,000 deaths and 103 billion DALYs are attributed to ambient air pollution worldwide (11). In 2014, approximately 83% of Europeans lived in areas above the World Health Organization (WHO) Air Quality Guidelines for PM_2.5, and PM_2.5 exposure was estimated to cause up to 428,000 premature deaths in 41 European countries (12). Moreover, people who migrate to industrialized nations such as the United States are more likely to be exposed to traffic-related air pollution than nonmigrants (13). Little is known about outdoor or indoor pollutants and respiratory health in migrants and refugees. In a Palestinian refugee camp, the prevalence and incidence of respiratory conditions (such as influenza, asthma, and bronchitis) were associated with poor housing conditions characterized by dampness and mold, dust and smoke, burning of biomass fuel, and crowding (14).

Tobacco use is the main cause of preventable death worldwide. Immigrants to the United States from Latin America and Asia generally smoke at lower rates than white and black adults in the United States, but this phenomenon varies by sex, acculturation, and national origin.

Immigrant men tend to smoke at higher rates than women: in 2015, only 7% of Latinas and less than 3% of Asian women reported current smoking (15). However, acculturation was significantly associated with 1.12 to 1.57 increased odds of smoking among Latinas in two nationwide studies. Among Latinos and Asians of both sexes, acculturation is associated with unchanged or lower smoking rates, but tobacco use differs by national origin. Puerto Ricans or Cubans and Vietnamese or Koreans smoke at higher rates than other Latinos or Asians, respectively (16, 17). In the Hispanic Community Health Study/Study of Latinos, in which approximately 85% of participants were migrants, current smoking was highest in Puerto Ricans (men, 35.0%; women, 32.6%) and Cubans, and lowest in Dominicans, with intermediate estimates for Mexican Americans (men, 23.4%; women, 10.4%) (16).

Other migrant groups to the United States bring tobacco use behaviors from their native countries. Middle Eastern and North African immigrants often use both water pipes and combustible tobacco, predominantly in men. For example, among adults in a mid-Western U.S. city, current use of both cigarettes and water pipes was 2.3 times higher in foreign-born Arab Americans than in whites (7.9% vs. 3.5%) (18).

Cigarette smoking is often managed as a nicotine addiction, but nearly one-half of immigrants who smoke report fewer than five cigarettes per day or nondaily use (16). There are no clinical trials of pharmacological treatment of very light or nondaily smokers, and few behavioral interventions have been adapted.

During the twentieth century and early in the twenty-first century, Western European countries received impoverished workers from Central and Southern Europe, Turkey, the Middle East, and Africa. In 2008, nearly 31 million foreign citizens (or ∼6% of the population) lived in the 27 states of the EU (19). Although nonuniform registration of migrants from different countries is a barrier to understanding migrants’ health in Europe (20), such migrants are often employed in jobs that are below their educational level and carry high risk of injury or disease, such as domestic work and cleaning; agriculture, horticulture, and forestry; hotel work, catering, and tourism; building and infrastructure; and transport.

Like their counterparts in Europe, immigrant workers in the United States are overrepresented in hazardous industries: agriculture, construction, and transportation. Moreover, immigrant workers have higher occupational fatality and injury rates than native workers (21). Although the overall occupational fatality rate in the United States has declined over the past two decades, the proportion of occupational fatalities among immigrant workers has markedly increased, likely due to lack of safety training and equipment, linguistic barriers, and precarious employment. This situation is even worse for undocumented immigrants, who risk losing work or deportation.

Data from specific industries illustrate the occupational risks of immigrants in the United States (22). In this country, approximately 60% of agricultural workers are immigrants who are at risk for noninfectious diseases (asthma, COPD, hypersensitivity pneumonitis, and interstitial fibrosis) and infectious respiratory illnesses (e.g., tuberculosis, which is more easily transmitted in crowded housing or work conditions). Moreover, immigrant women employed in household cleaning are at increased risk of asthma.

Childhood asthma is more common in high-income nations than in low–middle-income countries. After migrating to industrialized nations, however, children born in developing countries acquire a greater risk of asthma over time, likely due to environmental and lifestyle changes (i.e., reduced physical activity and a “Westernized” diet) (23, 24). For example, migrant children living in Italy for less than 5 years had 41% lower odds of lifetime asthma than native children (95% confidence interval for odds ratio, 0.23–0.66), but those living in Italy for at least 5 years had an asthma risk close to that of native children (25). Similar results were found in Australian adolescents (26) and in Swedish children or young adults (27). This migration-related asthma risk may be greatest for those migrating before age 5 years, as early life is critical to immune system development (28). Moreover, individuals born to migrant parents in high-income countries are at greater risk of asthma than their foreign-born parents: Mexican Americans born in the United States have twice the odds of asthma as those born in Mexico (29).

Adult migrants may not be as likely to develop asthma as migrating children, except for occupational asthma. In a 2001 study of adults living in 18 countries in the EU, there was no significant difference in bronchodilator responsiveness, atopy, or health care use for asthma between migrants and nonmigrants (30). In the United States, adult migrants are often employed as farm workers, which puts them at risk of chronic airflow obstruction and chronic bronchitis (31).

Little is known about respiratory diseases in refugee children. In a Palestinian study, children in refugee camps were at higher risk of asthma or asthma symptoms than those from neighboring villages or cities. Among refugee children, current wheeze (12.6%) and physician-diagnosed asthma (15.6%) were particularly common (32). Similar findings were obtained in a study of migrants crossing the Mediterranean Sea into Italy, in which young children (≤5 yr) and unaccompanied minors were particularly vulnerable to respiratory illnesses such as asthma and respiratory infections (33). Additional studies are needed to identify the relative contribution of risk factors for asthma among migrants and refugees.

Nearly 29 million people may have COPD in the United States (34). Latinos, whose ancestry can be traced to the former Spanish empire, are the largest minority group in the United States, constituting 17.6% of the nation’s population (35). Latino migrants are diverse for racial ancestry, country of origin, birthplace, and area of residence.

Consistent with patterns for asthma prevalence (36) and smoking, there are marked differences in disease prevalence among Latino subgroups, with COPD more likely in Puerto Ricans and Cubans than in Mexicans; such differences may be partly explained by the asthma-COPD overlap syndrome in Puerto Ricans. In the United States, foreign-born Latino immigrants often lack health insurance (∼55%), English proficiency, or high education, which affect health care access and quality (35).

Adequate sleep is vital to good health (37). Refugees commonly experience stress during and after relocation, along with traumas inducing them to migrate. Poor sleep after traumatic or stressful events is an independent risk factor for PTSD, impacting nearly 10% of refugees (a rate 10-fold higher than in the general population). PTSD itself further worsens sleep by causing insomnia and nightmares (the most refractory symptoms of this disorder) (38, 39). Inadequate resources have been invested in delivering effective behavioral treatments in languages spoken by refugees.

Immigrants in general also face stressors that adversely impact sleep, including discrimination, adverse neighborhoods, and high rates of shift work, underemployment, and unemployment. Nevertheless, immigrants often have better sleep than native-born residents of Western nations, perhaps due to lack of unhealthy sleep habits such as tobacco or caffeine use and screen time, or a belief system that values sleep. However, second-generation immigrants usually have worse sleep patterns than first-generation immigrants, possibly due to acculturation (40, 41).

Little is known about critical illness after unauthorized crossings along the U.S.–Mexico border. The Binational Migration Institute of the University of Arizona’s Mexican American Studies and Research Center comprehensively reports annual unauthorized border crosser (UBC) deaths (42). Although these reports have linked a “funnel effect” created by U.S. immigration control policies and a marked increment in known UBC deaths, such deaths are a poor surrogate for a spectrum of critical illness. A retrospective case review conducted at the University of Arizona School of Medicine–Tucson from 2010 to 2012 identified 55 admissions of UBCs to adult intensive care units, with one fatality (43). Several critical illnesses were identified, related mostly to exposure to a hot and dry environment, strenuous exercise, or dehydration.

The conflict in Syria has led to a humanitarian crisis and negative effects on health care (3), including critical care (44). In response to this crisis, some groups have used innovative methods to overcome lack of physicians and critical care expertise in a war-torn country (1, 44). In October 2012, Syrian-American physicians volunteered to train Syrian nurses on basic critical care skills in Turkey, and two nurses were initially chosen to start a remotely managed intensive care unit (ICU) in the governance of Idlib (1). Within 6 months, the program expanded to cover the ICU of every trauma hospital in eastern Aleppo, serving initially 2.5 million civilians with 11 to 15 remotely monitored ICU beds. Approximately 20 Arabic-speaking intensivists and other specialists from North America have volunteered to provide remote supervision and training to the nurses and medical students in the field. With their help, the program expanded to serve up to 40 ICU beds, using inexpensive communications technology (e.g., Internet protocol [IP] cameras and satellite Internet, and applications such as Skype and WhatsApp). The applicability and sustainability of this promising telemedicine model to other conflict zones have yet to be tested.

Communicable infectious diseases deserve attention when addressing migrant respiratory health; TB is perhaps the best studied, as it is most prevalent in low- and middle-income countries. TB incidence is increased by poverty, displacement, communal living, and malnutrition, circumstances commonly confronted by migrants and refugees. To address the worldwide impact of TB, the WHO developed a comprehensive strategy of TB control (the DOTS [directly observed treatment, short course] strategy) in 1995, focused on rapid diagnosis and treatment of infectious cases (45). Such approach then evolved toward the 2006 Stop TB strategy (including management of multidrug-resistant disease and TB in HIV-infected individuals), and the 2014 End TB strategy, which consists of three pillars: 1) technical interventions (prevention, diagnosis, and treatment), 2) policies and supporting systems (e.g., universal health coverage, social protection, and poverty alleviation), and 3) research activities (46). The End TB strategy, aligned with sustainable development goals, includes the concept of TB elimination (defined as <1 case per million population), which needs to be pursued by countries of low tuberculosis incidence (<10 cases per 100,000 population) through implementing eight core activities (44); of these, activity 3 focuses on the needs of migrants and transborder issues, while activity 4 deals with the diagnosis and treatment of latent infection (47).

An ERS/WHO survey in Europe, besides presenting important country-specific differences, revealed that much less programmatic information is available for latent infection than for TB disease (48). A 2016 statement called for proper implementation of the End TB strategy, improved surveillance, timely screening, stigma prevention, universal access, and adequate infection control (49). Research evaluating best practices to accomplish the above goals throughout the affected regions, and the cost-effectiveness of such interventions, is needed while south–north and east–west migration movements continue.

Efforts to screen and treat TB in migrants and refugees are of the utmost importance. In 2015, TB notification in the EU and European Economic Area (EU/EEA) countries was 11.7 per 100,000 population (range, 2.1–76.5), with an average of 29.8% of cases in persons of foreign origin (range, 0.2–89.5%) (50). In countries with a high proportion of TB cases in migrants, screening for active TB and/or latent TB infection (LTBI) is key to TB elimination (51–53). However, the actual implementation of TB/LTBI screening of migrants differs substantially in the EU/EEA (53). Published studies on the yield of migrant screening are so heterogeneous that no valid conclusions can be drawn on the most effective approach. Moreover, few EU/EEA countries have health information systems that can capture results of migrant TB/LTBI screening and evaluate the screening systematically. This has led to the identification of several research needs and initiatives, of which one is the E-DETECT TB project, cofinanced by the European Commission, which aims to use country screening data and develop a standardized protocol to collect indicators, share data, and build better systems at the national and international levels, with a long-term goal to initiate future routine reporting into a European database (54).

Limited information and standardization are also available regarding the screening and management of TB/LTBI among U.S. migrants, which depends on personnel and other resources. Refugees resettled in the state of Kentucky, for example, arrived from more than 30 different countries. The overall rate of LTBI among migrants and refugees resettling in Kentucky is 9% versus 5% among native-born U.S. citizens (55). Of the 581 refugees arriving between January 2013 and December 2014, 45 were eligible for treatment of LTBI, but only 12 (27%) completed treatment. Primary reasons for nonadherence to treatment included language barriers, work schedule conflicts, the perception that treatment was unnecessary, and difficulties in accessing medication. Key interventions implemented included 1) stakeholder meetings with representation from the Kentucky Office for Refugees, the local health department, and academic leaders; 2) opening of a clinic focusing on treatment of LTBI; and 3) use of global health navigators as key personnel in centering care around the needs of the individual refugee. However, the impact of those interventions remains unclear.

Another important infectious disease in migrants and refugees is HIV/acquired immune-deficiency syndrome (AIDS). Although it is commonly believed that mobility, conflict, and displacement are associated with HIV transmission, the evidence that this translates into more infections is not uniformly supported by published data (56). Risk factors for HIV infection in migrants and refugees include breakdowns in social structures, sexual violence and abuse, and inadequate access to health care (56). However, what is often not assessed are protective factors against HIV transmission, such as decreased mobility, and even improved circumstances in some of the refugee camps regarding health, education, and social conditions. Important also are the HIV prevalence rates in the affected and in the surrounding host communities, as well as their interaction (56). The Office of the United Nations High Commissioner for Refugees, together with its various partners such as the United Nations Program on HIV/AIDS, has published guidelines regarding the rights of refugees and best practices to provide HIV-related services to both refugees and the surrounding host country populations (57). Refugees and internally displaced persons can achieve high adherence to antiretroviral therapy with associated viral suppression (58).

HIV infection in deportees is an emerging area of interest. Since 2007, more than 319,000 migrants, predominantly Mexican, have been deported annually from the United States (59). Such statistics have motivated research on the socioeconomic and health implications of deportation, especially as families are separated and communities on both sides of the U.S.–Mexico border become destabilized. Migrants’ forced displacement may result in the loss of social and economic resources (i.e., jobs), which may adversely affect the deportees’ health (59–62). Studies conducted in a border metropolis (Tijuana, Mexico) have shown that deportees are at high risk for HIV infection (63).

Pulmonary disease due to nontuberculous mycobacteria (NTM) is another increasingly prevalent condition (64). Although our understanding of this disease has improved, published evidence on its impact in migrants and refugees is minimal and largely restricted to case reports (65–67). Insufficient research on NTM in migrants may be explained by heterogeneous data on NTM-pulmonary disease (PD) epidemiology, in light of the geographic diversity of the NTM species (68); the fact that NTM-PD remains a not-reportable disease in most countries; and lack of concern for human-to-human transmission of NTM-PD, with conflicting evidence for possible transmission of Mycobacterium abscessus among patients with cystic fibrosis (69, 70).