Link found between air pollution and asthma hospitalizations

Smoke gets in their eyes and lungs

Air pollution may be a little-known trigger for acute asthma exacerbations, according to several recent studies. Researchers at the University of Washington in Seattle found asthma hospitalizations increase by as much as 6% at times when levels of airborne particles and carbon monoxide were high. Other studies had similar findings:

• A report in Santa Clara County, CA, showed a relationship between increasing levels of smoke from wood stoves and emergency department (ED) visits.

• Researchers at Brigham and Women’s Hospital in Boston found peak expiratory flows diminished by more than 7% during the typical school day in children monitored in heavily polluted Mexico City.

• European studies also show sulphur dioxide triggers acute asthma attacks.

A concern for severe asthmatics

It all adds up to a simple conclusion, says David Mannino, MD, a medical epidemiologist at the Centers for Disease Control and Prevention in Atlanta. "Doctors should consider that air pollution is clearly related to hospitalizations and exacerbations in people with severe asthma."

He says the 15% of diagnosed asthmatics categorized as "severe" probably already know they "should be paying attention to outdoor air quality." Particularly, Mannino says, severe asthmatics should be aware that ozone levels are higher in the afternoon and early evening on warm days, so "they just don’t go outside, and particularly they shouldn’t exercise at these times."

"There is evidence out there that some air pollutants are worse than others and might make an individual with asthma more likely to have an attack," says William McDonnell, MD, PhD, a medical officer with the U.S. Environmental Protection Agency in Chapel Hill, NC.

Asthma patients are "very, very sensitive" to sulphur dioxide, says McDonnell "because it causes airway constriction at a level much lower than would cause such a response in the general population."

Airborne particles

There are two culprits considered to be major contributors to the elevated levels of particulate matter linked to asthma attacks: wood smoke and vehicle exhaust.

"The smaller the particles, the farther they get into the lungs," says Lianne Sheppard, PhD, research assistant professor in the department of biostatistics at the University of Washington in Seattle and lead author of a study published in the January issue of Epidemiology.

Sheppard says asthma patients are among the "sensitive population out there that need to be protected under the Clean Air Act."

Since Washington’s state laws clamped down on wood stove burning a few years ago, she says, vehicle emissions are "more likely the cause" of the rise in asthma hospitalizations during times of intense increases in small particulate matter, 10 microns or less in diameter.

The California study covered roughly the same time frame but focused on asthma-related ED visits during winter months. Researchers found that as temperatures dropped, ED visits rose, a statistic they attribute to increasing wood smoke concentrations in the air. Several states, including Washington, Vermont, and Colorado, have restricted or outright banned wood burning because of the aggravating effects on those with lung and cardiovascular conditions.

As Sheppard notes in her study, she didn’t see fewer people getting sick as the air improved from the controls placed on automobiles and wood burning.

Sheppard found the largest increases in hospitalizations were not in the winter, with its peak wood burning time, but in the spring and fall, "times when people are more likely to be outside and perhaps engaged in exercise." She compiled air pollution data from weather service temperature records and pollution monitors located in various parts of the city of Seattle from 1987 to 1994 and hospital admission records by diagnosis code for patients under 65 during the same time period.

She found more than half (54%) of asthma hospitalizations during peak pollution periods were in children and adolescents under 19.

Her team also noted a September peak in admissions among children, but discounted its relationship to air pollution because of its correlation to the return to school and increased incidence of upper respiratory infections common at that time of year.

Her study found little correlation between sulphur dioxide levels and asthma hospitalizations in Seattle, which has relatively low SO2 levels, but other studies have shown the two are related."I think in terms of triggers, overall, this is pretty small, but it is something to be aware of," Sheppard says. "It could be in the background and be important in a less-obvious way."

She thinks, as a population, Americans are becoming more sensitive to environmental pollutants, "maybe because we’re not subjected to infectious diseases as much as we used to be, so people are susceptible to lower trigger levels." The mushrooming rate of serious asthma cases, she says, "may be because people are not dying for other reasons or perhaps they are being diagnosed with asthma at an earlier age."

"There are a number of things out there that might make asthma worse, and air pollution is certainly something we’re watching," McDonnell says. Yet he praises the Clean Air Act for its effectiveness: "Look at Los Angeles over the last 20 years. The ozone concentrations and other pollutants are coming down dramatically."

[David Mannino can be reached at (770) 488-7313. Contact William McDonnell at (919) 966-6220 and Lianne Sheppard at (206) 616-2722.]