SAN FRANCISCO—Although many of the pollution sources now recognized as threats to respiratory health arose during or after the Industrial Revolution, some are much older: Vegetation burned either in wildfires or in the household for cooking or heating can significantly contribute to respiratory disease. In a session at the American Thoracic Society 97th International Conference, researchers gathered to review the epidemiology and biology of smoke’s effects on respiratory health, as well as possible strategies for mitigation.

“[I]t is difficult to burn wood completely in simple stoves,” explained Kirk R. Smith, MPH, PhD, Professor and chair of the Division of Environmental Health Sciences in the School of Public Health, University of California, Berkeley.[1] “[P]roducts of incomplete combustion are a vast range of chemicals, many of which … cause ill health: small particles, carbon monoxide, nitric oxide, and PAHs [polycyclic aromatic hydrocarbons]. Almost all the particles respirable are below one micron [in size],” allowing them to deposit deep within the respiratory tract.

“In the developed world, exposure to smoke comes primarily from fireplaces and wood stoves, outdoor burning for land clearing, and agricultural and forest fires,” said Jane Q. Koenig, PhD, Director of the EPA Northwest Center for Particulate Matter and Health.[2] Dr. Koenig, a Professor of Environmental Health at the University of Washington, Seattle, cited evidence that agricultural burning can increase concentrations of particulate matter less than 10 µm in diameter (PM10) several miles away.

The increased PM10 concentrations can adversely affect human health, she noted. Studies have shown that there is a direct correlation between emergency room (ER) visits for asthma and the number of acres of rice-seed–stubble burned. In a study at the University of California, San Francisco, controlled exposures to rice-seed–stubble smoke generated in the laboratory led to an increased number of alveolar macrophages in bronchoalveolar lavage fluid.

Michael Brauer, ScD, Associate Professor of Occupational and Environmental Hygiene at the University of British Columbia in Vancouver, further delineated the impact of uncontrolled fires, using as an example a series of wildfires that occurred in Indonesia in 1997.3 In a one- to two-month episode, PM10 concentrations reached 400 µg/m3 in Kuala Lumpur, 300 to 1,000 km away from the fires.

“On some days, there were 90 million people in this region exposed [to levels] above 150 µg/m3,” said Dr. Brauer. He also noted that the number of ER visits for respiratory symptoms at Kuala Lumpur General Hospital rose threefold during sharp elevations in the PM10 concentration. As a result of the same uncontrolled Indonesian fires, Dr. Brauer said, “In Singapore, an increase in PM10 concentration was associated with increases in outpatient visits for asthma, upper respiratory illness, [and] rhinitis.”

The impact of the Indonesian wildfires was specifically examined in a cohort of children monitored for forced expiratory volume and forced vital capacity. Not only did these measurements drop during the fires, he noted, but also “measurements [obtained] six months and one year later still did not show a full return to baseline lung function, … which is suggestive of chronic effects.”

Michael Lipsett, MD, JD, an Associate Clinical Professor at the University of California School of Medicine, described the health effects of wildfires that occurred in northern California in the fall of 1987,[4] during which air particle measurements peaked at more than 4,000 µg/m3. “We looked at 15 hospitals in the six counties … most affected by the smoke,” he said. During the fires, Lipsett et al found a “consistent picture of increased [ER] visits for asthma, COPD, [and] both upper and lower respiratory conditions.” Dr. Lipsett also cited findings obtained during the 1991 Berkeley/Oakland Hills fires, when wildfires occurred close to an urban population: “About 30% of the visits to the ERs and most of the hospital admissions were for bronchospastic reactions.” Most of the patients had a prior history of asthma or COPD, Dr. Lipsett said. However, people who were ostensibly healthy were also at risk, he added. The timing of effects relative to the fire was also revealed: On average, ER visits and hospital admissions lagged from a day to a day and a half behind the day of the fire event. Dr. Lipsett noted a 91% increase in asthma visits and exacerbations of chronic bronchitis during a central Florida fire in 1998.

Dr. Brauer stated that the World Health Organization (WHO) has spearheaded efforts to provide countries affected by outdoor fires with advice for limiting health effects. In addition to encouraging fire prevention, WHO’s recommendations for reducing their health impact include remaining indoors if air-conditioning or filtration is possible, using air cleaners, and reducing physical activity. WHO’s action plan also recommends precautionary measures for people who have to be outdoors and the establishment of emergency shelters with filtered air, especially for people who may be susceptible to the effects of air pollution.

The Centers for Disease Control and Prevention (CDC) has also been investigating this issue. Dr. Lipsett described a recent study conducted by the CDC, which involved a retrospective survey of intervention efficacy during a fire that lasted about two months in the fall of 1999 on the Hoopa Valley National Indian Reservation in northern California. During the fire, “PM10 concentrations were elevated, on occasion exceeding 500 µg/m3”; this was associated with “an increase in respiratory visits to the local clinic by 52% over the [number of visits during] the prior year,” he noted.

The CDC searched for fire-associated changes in self-reported symptoms in groups receiving a variety of interventions: masks; vouchers to go to hotels on the coast, out of the smoke; high-efficiency particulate air (HEPA) filters; and public service announcements telling people to stay indoors, to use masks, and to evacuate, if necessary.

The CDC’s Hoopa Valley survey revealed that two strategies were effective, according to Dr. Lipsett: “HEPA filter use was associated with a lower odds ratio for reporting worsening respiratory symptoms.” Residents who could remember hearing public service announcements also had reduced odds, he said.

Dr. Brauer commented, “The most common measure that is used is masks, [but] they’re not always used that effectively. [Standard N95 masks work] by electrostatic interactions with particles. [T]hese masks, when fitted properly, will filter more than 90% of the particles. The problem is with the fit, compliance, and use.” In the Hoopa Valley CDC survey, said Dr. Lipsett, “Mask use was not associated with symptom reduction. [If] the masks don’t fit well, … they’re not going to be effective either against gases or against particles, [and they may] give people a false sense of security.”

While scant resources exist in developing countries to reduce household wood smoke exposure, “the American Lung Association (ALA) has recognized that residential wood smoke is, in fact, a problem,” said Judith T. Zelikoff, PhD, Associate Professor of Environmental Medicine at New York University School of Medicine in New York City.[5] The ALA recommends that homeowners in this country “convert … wood-burning fireplaces to use natural gas or propane, [and] certainly to replace any stoves made before 1988 with EPA-certified equipment.”

—Mimi Zucker, PhD

Where There’s Smoke … There’s Increased Infection Risk

In developing countries, the use of wood or other solid organic fuels for cooking or heating is a major threat to respiratory health, particularly in children. Not only does it exacerbate preexisting lung disease, but it also increases the risk of respiratory infections. For example, said Kirk R. Smith, MPH, PhD, about 80% of households in India use solid fuel, and there is strong evidence that this adversely affects the health of women and small children. Based on several dozen epidemiological studies in developing countries, about 500,000 premature deaths a year seem to be due to indoor air pollution from burning vegetation in India. Much of this impact occurs through the enhancement of respiratory infections. How does smoke exposure increase the infection risk? Judith T. Zelikoff, PhD, explained: “[U]sing wood smoke and rats … we’ve shown that there’s an increased susceptibility of smoke-exposed animals to pulmonary infection with the opportunistic bacterial pathogen, Staphylococcus aureus.” Dr. Zelikoff demonstrated that “clearance of the bacteria [from the lung] was … dramatically reduced … following exposure. By five days post-exposure, exposed rats were only able to clear about 10% of the infectious agent.” To better understand the mechanism by which this effect upon pulmonary clearance may occur, Dr. Zelikoff also examined the ex vivo responses of macrophages from the lungs of wood-smoke–exposed animals. “In addition to reduced intracellular killing of the bacterial pathogen, macrophages had reduced ability to engulf opsonized particles and produce superoxide anion, a critical cytotoxic factor necessary for intracellular killing of S aureus,” she said. —Mimi Zucker, PhD

Sources: Smith,[1] Zelikoff.[5] American Thoracic Society 97th International Conference. 2001.

References
1. Smith KR. Comparison of methods to determine burden of disease from biomass smoke exposures in developing countries. Presented at: American Thoracic Society 97th International Conference; May 22, 2001; San Francisco.
2. Koenig JQ. Exposures to emissions from biomass cooking and heating fuels and associated health effects in the developed world. Presented at: American Thoracic Society 97th International Conference; May 22, 2001; San Francisco.
3. Brauer M. Exposures to air pollution from vegetative fires and associated health effects: an international perspective. Presented at: American Thoracic Society 97th International Conference; May 22, 2001; San Francisco.
4. Lipsett M. Health impacts of exposures to wildfire smoke in the United States. Presented at: American Thoracic Society 97th International Conference; May 22, 2001; San Francisco.
5. Zelikoff JT. Possible mechanisms of adverse respiratory health effects from woodsmoke exposure. Presented at: American Thoracic Society 97th International Conference; May 22, 2001; San Francisco.