Urban Medicine: Threats to Health of Travelers to Developing World Cities

Christopher Sanford, MD, DTM&H

In This Article

Air Pollution

The first attempt to control air pollution occurred in 1306, when England's King Edward I banned the burning of coal in an effort to control the malodorous clouds of coal smoke above London. The ban was not enforced, and London became one of the first cities to suffer from significant air pollution.[59]

It is not necessary to memorize which foreign cities have significant air pollution; it is safe to state that virtually all large cities in the developing world have significantly polluted air. In 1992, Mexico City was deemed by the World Health Organization to have the most polluted air in the world. However, over recent years, through regulatory control, the air of Mexico City, while still heinous, has significantly improved. Currently, China has the unwelcome distinction of having nine of the 10 cities with the worst air pollution in the world.

The developing world has no monopoly on polluted air. In 1880, 2,200 Londoners were killed by a toxic smog of coal smoke. The Meuse Valley Fog of 1930 killed over 60 people in Belgium.[60] In 1948, 50 resident of Denora, Pennsylvania were killed by a "killer fog".[61] In December 1952, the "Great Smog" of London brought the capital to a standstill and caused an estimated 4,000 acute deaths; an additional 8,600 excess deaths among Londoners occurred during the first 3 months of 1953.[62] The ongoing smog of Los Angeles is well known. However, the air of urban centers in developed nations seems pristine and alpine when compared to that of large cities in the developing world. For example, the levels of total suspended particulates in micrograms per cubic meter for Stockholm, Mexico City and Lanzhou, China are 9, 279, and 732, respectively. Levels of sulfur dioxide, which is formed through the burning of fossil fuels such as oil or gas, in micrograms per cubic meter, in Los Angeles, California and Guiyang, China are 9 and 424, respectively.[63] In Jakarta, Indonesia, due to most vehicles using leaded gasoline, the level of lead in the atmosphere in 2000 was 1.3µg/m3; the World Health Organization's recommended maximum is between 0.5 and 1.0µg/m3.[64,65] In Bangkok, motorcycles are popular, as they can thread through the ever-present traffic jams. Between 1993 and 2000, the number of registered motorcycles in the Bangkok metropolitan region increased from 1.1 million to 2 million;[66] future growth is projected to be 15%/year. Ninety percent of these motorcycles have two-stroke engines, which are the worst offenders with regard to suspended particulate matter and hydrocarbon emissions.[67] The numbers of vehicles in Asian urban areas have grown exponentially over the past two decades; in Delhi and Manila, the number of vehicles has been doubling every 7 years.[68]

The United States Environmental Protection Agency terms the six principal air pollutants "criteria pollutants"; these are carbon monoxide, nitrogen dioxide, ground-level ozone (not to be confused with "good ozone", which is in the stratosphere at 10 to 50km above the earth), particulate matter (airborne particles <10µm in diameter), sulfur dioxide, and lead. Some medical writers have tried to compare the level of air pollution in a given city with smoking a certain number of cigarettes per day, but this is an inaccurate analogy; the pollutants and carcinogens are different.

Carbon monoxide is formed by the burning of fuels such as gasoline, oil, and wood. Persons with pulmonary and cardiac disease may develop dyspnea and angina at carboxyhemoglobin (COHB) levels of 3% to 4%. Exercise in a traffic tunnel will increase the COHB level to 5% within 90min.[69] Elevated carbon monoxide levels have been found to increase the rates of hospitalization of elderly patients with congestive heart failure.[70]

The effects of air pollution, specifically ozone and particulate matter, on mortality and hospital admissions due to respiratory and cardiopulmonary disease have been found in both short-term studies, which have investigated day-to-day variations of pollutants, and long-term studies, which have followed cohorts of urban residents over some years. Effects have been found even at very low levels of exposure; hence, it is unclear whether or not a threshold value exists below which no effects on health are noted.[71]

Air pollution does not have to be severe to impact on children with asthma. In a French study, children with mild-to-moderate asthma had measurable short-term decrements in pulmonary function tests that correlated with prevailing levels of photo-oxidant and particulate pollution, even though the levels of these pollutants were within those specified by international air standards.[72]

Air polluted with particulate matter causes increased serum concentrations of fibrinogen and platelets, with sequestration of red blood cells in the lungs.[73] Particulate matter also increases the risk of cardiac arrhythmias,[74] but the significance of these changes for cardiovascular events remains unclear.[75]

Multiple studies have linked air pollution to increased mortality.[76] A restriction that reduced the sulfur content of fuel oil utilized by power plants and road vehicles in Hong Kong led to a substantial reduction in deaths from all causes, from respiratory diseases, and from cardiovascular diseases.[77] Long-term exposure to fine particulate air pollution causes increased cardiopulmonary morbidity, including lung cancer.[78] A study of the 1997 "haze disaster" in Indonesia found that over 90% of 543 people interviewed had respiratory symptoms. The elderly, and those with a history of asthma, had increased symptoms; wearing a mask correlated with lessened symptoms.[79]

Ozone is produced by the effect of sunlight on volatile organic compounds or oxides of nitrogen. In a study of children who performed in outdoor team sports in 12 communities in southern California with varying levels of pollution, ozone was the pollutant most strongly associated with the development of asthma.[80] Ozone triggers inflammation, and animal studies suggest that it causes increased susceptibility to bacterial infection. Great variability among individuals exists regarding the response to ozone, with a minority demonstrating significant responses at only moderate levels. An individual's sensitivity to ozone appears to have a genetic basis.[81] For each 50 parts per billion increase in peak ozone levels, hospitalization rates increase by 6% to 10% for asthma, pneumonia, and chronic obstructive pulmonary disease (COPD). Children living in Santiago, Chile were found to have increases in lower respiratory tract illnesses in direct proportion to the levels of particulate matter and ozone.[82] For children 3 to 15 years of age, the increase in lower respiratory tract symptoms is 3% to 9% for a 50µg/m3 change in particulate matter, and 5% for a 50 parts per billion change in ozone. Intriguingly, the use of antioxidant vitamin supplementation has been found to markedly reduce the ozone-induced reduction in pulmonary function in young, healthy, nonsmoking adults.[83]

A study performed in India found that urban children had a higher level of exercise-induced bronchospasm than did children living in rural areas.[84] A recent study performed in southern California, US, found that associations between severity of air pollution and asthma were stronger in asthmatic children not taking anti-inflammatories (inhaled cromolyn, nedocromil sodium, or corticosteroids) than in children who were.[85]

A Denver (Colorado, US) study found strong associations between rates of childhood cancers and leukemia, and distance of residence from streets with a high density of motor vehicles. In the highest traffic density category, that of close proximity to roads with more than 20,000 vehicles/day, the odds ratio for all cancers was 5.90, and that for leukemia was 8.28.[86] In Amsterdam, a much higher relative risk of death was found in individuals who lived on main roads, as compared to those who lived away from main roads.[87]

Recent epidemiological research indicates that the effects of air pollution on life-expectancy are not uniformly distributed throughout populations, but are influenced by factors including education level and antioxidant vitamin status.[80,88,89] During an exacerbation of air pollution in Augsburg, Germany in 1985, increases were noted in residents' heart rate, plasma viscosity, and C-reactive protein, all of which can contribute to an increased risk of cardiovascular events.[90,91,92]

Given that air pollution has been found to affect residents' health in both short-term and long-term studies, there is no reason to think that travelers are not impacted by it. However, the nature and extent of the risk are yet to be determined.

What does this mean for the travel provider? Patients with COPD should travel with a "rescue cocktail" of three drugs for use during exacerbations: an additional bronchodilator, an oral steroid, and an appropriate antibiotic. Patients with asthma, particularly those with a history of exacerbation in response to air pollution, should carry an additional inhaler and an oral steroid. Antioxidant vitamins may reduce the decrement in pulmonary function of travelers to cities with severe air pollution,[82] but controlled studies are lacking. Placing asthmatic children on an anti-inflammatory medication, e.g., montelukast sodium (Singulair), just prior to and during their stay in a heavily polluted region would be reasonable and possibly of benefit. For elderly travelers, a pretravel physical examination with stress treadmill and pulmonary function tests may be useful in screening for cardiovascular and pulmonary disease.[93] Certainly, travelers with cardiac or pulmonary disease should have these conditions adequately controlled prior to departure.

It is reasonable to assume that long-term visitors to heavily polluted cities will develop many of the same sequelae as do residents. The effects of air pollution on the short-term traveler have not been studied; anecdotally, many develop eye and respiratory irritation. Travelers with pulmonary or cardiac disease should be advised to minimize their duration of stay in heavily polluted cities, to avoid heavy exercise while residing therein, and to have a low threshold for seeking medical care should they become dyspneic or develop chest pain. The International Association for Medical Aid to Travelers (IAMAT), founded in 1960, is a Canadian-based nonprofit organization that offers a list of IAMAT-affiliated physicians around the world, all of whom speak English and have trained in North America or Europe.[94] Consideration should be given to writing prescriptions for "rescue" medications for patients with COPD or asthma, including an oral steroid, an appropriate antibiotic and an additional bronchodilator as standby medications for exacerbation.