Air Pollution and Asthma



street smoke MR 2/2000

Tiny Particles Take a Big Toll

By Tracy L. Schmierer

Invisible particles hang in wait, primed to spread their toxins to the next victim who dares to breathe, laugh or sigh. Despite efforts to clean up our air, pollution remains a serious health problem.

Research indicates that air pollution is associated with premature mortality and an increased number of hospital admissions, emergency department visits and doctor appointments, says George D. Thurston, ScD, associate professor of environmental medicine at the New York University School of Medicine, Tuxedo, N.Y. Specifically, particulate matter and ozone air pollution can cause asthmatic reactions.

“Asthmatics’ freedom to go outside is being infringed upon by air pollution, and that’s not right,” Dr. Thurston says.

In order to liberate their patients, clinicians and researchers are studying which air pollutants are the most dangerous, how they exacerbate asthma, and who is most at risk.


Air pollutants can be divided into three categories, according to Ted Schettler, MD, science director for the Science and Environment Health Network, and specialist, East Boston Neighborhood Health Center. They include:

* biologicals, such as cat dander, dust mites, pollen and molds, which all have some kind of biological source

* chemicals, such as latex, or other industrial chemicals found in the workplace or in consumer products, which can cause occupational asthma

* combustion byproducts, such as diesel exhaust, ozone, sulfur dioxide, and particulate air pollution, which are found in the ambient air.

The majority of these air pollutants usually are found in urban environments, Dr. Schettler points out. He says sometimes rural ozone levels are extraordinarily high, but urban environments are more likely to contain the complex mixtures that create the particles responsible for the problem, such as latex that comes off rubber car tires. “There’s evidence to suggest that the aerosol of latex particles may contribute to the increased incidence of asthma in urban environments, which are near highway systems.”

Because pollution is a composite of many types of particles, it’s difficult for researchers to pinpoint which are the most hazardous. Current research focuses on concentrated, fine particle compounds–including lead, sulfuric acid and arsenic–that are small enough to get into the trachea and down into the lungs. “We set a standard PM10, which is 10 microns in diameter or smaller,” Dr. Thurston explains. “A human hair is about 100 microns in diameter.”

These tiny molecules take a big toll. For example, if you look at the toxicity of fossil fuel combustion on a pound for pound basis, air pollution is an “order of magnitude more toxic than tobacco smoke,” Dr. Thurston says. “One of the big questions facing science is what is it about particles that give them this toxicity, especially ambient particles from fossil fuel combustion, which seem to be very toxic.”


The way in which these pollutants harm asthmatics is by acting as airway irritants, says John Wright, RRT, MBA, BS, assistant director of respiratory care at the UCLA Medical Center in Los Angeles. “The airways clamp down, and a small change in airway diameter can have a significant change in flow rates. And they can have tremendous difficulty in breathing,” he explains.

Dr. Thurston’s research also links air pollution to reduced lung function and the ability of the lung to exhale air. “Ozone is an irritant gas and can induce asthma attacks on its own at higher levels. But at lower levels, it seems to be playing a role of a promoter of asthma attacks, due to other causes.”

The mechanisms by which various contaminants can cause or exacerbate asthma are quite varied, Dr. Schettler says. For example, when someone allergic to cat dander comes into contact with it, they can have an allergic response and start to wheeze. Air pollutants also can act as irritants instead of allergens, causing bronchospasms and wheezing. A third possibility is the pollutants can aggravate the allergic response, in that they can boost the immune response so it increases the sensitivity of the individual to allergens.

Recent studies on diesel exhaust particles show they contain aromatic hydrocarbons that dissolve into the cells and interact with genes, causing them to produce more allergen-specific IgE, reports Michelle Lierl, MD, adjunct associate professor of clinical pediatrics in the division of allergy-immunology and pulmonary medicine at Children’s Hospital Medical Center in Cincinnati.

Another study measured allergen-specific IgE levels through baseline washings from the nasal mucosal of allergic rhinitis patients, she said. Researchers did a ragweed challenge using plain ragweed and ragweed plus diesel exhaust particles. They found much higher levels of anti-ragweed IgE were produced with the ragweed/exhaust combination.

Dr. Lierl’s research shows some allergic reactions eventually translate into asthmatic hospitalizations. From April through October 1996 and 1997, the number of asthma-related ER visits at her facility correlated best with pollen counts and were greatest three days after a high pollen count day. She suggests that clinicians control for pollen in future studies on other air pollutants so studies aren’t influenced by this predicting factor.


Pollution seems worse for pediatric asthmatics, possibly because they’re more active than adults, thus breathing more air per pound of body weight. They breathe in bigger doses of air pollutants and tend to get higher toxin levels. There’s also a bigger pool of asthmatics in the pediatric age group, so more are affected by air pollution, Dr. Thurston says.

Dr. Schettler agrees, adding that children often spend more time outdoors, which could lead to more exposure. Asthmatic children who play outdoors on high ozone-level days are 20 percent to 40 percent more likely to have an exacerbation, reports the greater Boston area Physicians for Social Responsibility, a medical organization Dr. Schettler belongs to that is committed to improving health through a cleaner environment.

Another theory suggests early life exposures, including fetal exposures to substances such as environmental tobacco smoke, could permanently alter the immune system.

“This would make the child more susceptible to the development of allergic asthma as a result of early life exposure,” Dr. Schettler offers. And the combination of increased exposure and increased susceptibility for children could decide if air pollutants will have short-term or lifelong effects on the asthmatic.


Yet air pollution has been significantly reduced over the years, thanks to social consciousness about air pollution. The Clean Air Act of 1990, among other standards, requires that cars meet carbon monoxide standards and have routine emissions control tests.

Los Angeles, a city infamous for its heavy traffic, high ozone levels and smog, “was at its very worst in the ’70s,” Wright says. “There were times when the sky was very dark from air pollution.” However, today Los Angeles citizens drive cars with emissions control, and their factories have adopted stringent air quality guidelines. And like most high-pollution cities, the local news gives a daily rating of the air quality to warn residents of high pollution or ozone days.

Dr. Thurston says reducing exposure to pollutants and taking medications, such as inhaled corticosteroids, albu-terol and beta2-agonists, and using bronchodilators and inhalers, can reduce the adverse health effects of air pollutants. However, he recommends people should become involved with groups that favor cleaner air, such as the American Lung Association, and support asthmatics by standing behind the implementation of new industry standards.

There is still much to be done. Many physicians don’t recognize pollution’s role in their asthmatics’ hospitalizations but instead list asthma as the cause of the reaction.

“Air pollution is not one of the ICD-9 codes, and it’s not one of the skin tests physicians can do to see what’s causing asthma problems,” Dr. Thurston states. “It’s really an underreported, underrecognized problem with asthma.”

Tracy Schmierer is assistant editor of ADVANCE.