Fire, Human Health, and Air Quality

Forest fires produce biomass smoke containing pollutants that have potential adverse effects on human health. Adverse effects of biomass smoke are defined as medically-significant or culturally-recognized biophysical or psychosocial changes in individual or population health (American Thoracic Society 2000). In addition to adverse effects, beneficial changes in interpersonal relations, and in socio-cultural, economic, and political systems can occur as a consequence of forest fires.

Medically significant biophysical effects of biomass smoke include acute, subchronic, and chronic effects on public health.The major constituents of biomass smoke (Table:Human Health Effects of Selected Smoke Constituents from Biomass Smoke)have adverse biophysical effects. The spectrum of adverse physiological affects ranges from temporary, relatively minor eye, nose, and throat irritations, to persistent cardiopulmonary conditions and less-commonly, to premature death. The most notable subset of biophysical effects involves heart and lung conditions (Table:Heart and Lung Conditions Associated with Biomass Smoke). It is clear that air pollution in general interferes with heart and lung processes. Changes in heart and lung processes caused by biomass smoke as one specific type of air pollution are not as clear. There are limitations to this research. For instance, fire effects are variable.

Smoke Inhalation Hazards

Inhalation, ingestion, and dermal absorption are the exposure routes of biomass smoke pollutants. Inhalation is the most common pathway through which humans absorb constituents of biomass smoke. Dermal absorption might also occur through a person’s surface (or skin) cells. One substance that skin cells directly absorb is free radicals that may contribute to the development of emphysema, Adult/Acute Respiratory Distress Syndrome (ARDS), and lung cancer (Dost 1991). Gastrointestinal absorption is another pathway of exposure to the pollutants emitted by fires. Gastrointestinal absorption can occur through the ingestion of products such as plants that have absorbed pollutants through the soil or ash, wildlife that have inhaled or ingested pollutants, and freshwater species (e.g., fish) that have absorbed or ingested contaminated water.

Impaired Visibility Hazards

The principle connection between visibility and human health is that the reduction of visibility due to forest fire smoke can cause highway vehicle accidents leading to injuries and fatalities (Goh and others 1999; Schwela and others 2000). Another connection between visibility and human health is that forest fire smoke reduces the aesthetics of a vista that can have psychological consequences for people who value clear views. The cultural preference for scenic vistas that many Americans share is considered to be an Air Quality Related Value (AQRV) (Tonnassen 2000). The reductions in visibility that sometimes accompany biomass smoke can change the look of the landscape, typically in ways that do not coincide with human preferences. People have more appreciation for the beauty of landscapes when their views are unobstructed by smog (Machlis 2002).

Governmental regulations require fire personnel to maintain air quality and visibility. Fire workers are specifically trained to manage smoke so that the general public encounters minimal amounts. Researchers in the USDA Forest Service and other fire agencies devote a great deal of attention to understanding smoke and devising techniques to control it during wildfires and prescribed fires. Unfortunately, there are cases where fire behavior, meteorology, and population patterns make smoke management very difficult. The degree of visibility reduction in any area depends on the character and concentration of smoke emitted by a forest fire combined with meteorological factors such as humidity, atmospheric stability, and wind patterns. Visibility decreases as humidity rates increase because more water is available for particulate matter to absorb which increases the ability of particulates to scatter light (EPA 1998). The high humidity that is typically found in many parts of the South results in nuisance smoke being more frequent in this region than in some other regions of the United States. High humidity combined with other factors make nuisance smoke a particularly important issue in the South. Achtemeier (2002: 41) describes the complexity of the situation as follows: “Meterology, climate, and topography combine with population density and fire frequency to make nuisance smoke a chronic issue in the south.”

The Environmental Protection Agency (EPA) considers visibility to be a matter of “public welfare” (EPA 1998). To protect public welfare, the EPA has established primary and secondary National Ambient Air Quality Standards (NAAQS) with the goal of maintaining socially acceptable levels of visibility. The Interagency Monitoring of Protected Visual Environments (IMPROVE), a coalition of EPA employees and federal land managers, monitors and enforces compliance with NAAQS. State, tribal, and local laws also contribute to the regulation of “nuisance smoke,” a category that includes the smog that limits visibility. Resource management organizations, timber companies, and private landowners cooperate with governmental agencies in fire and smoke management activities (Mutch 2002).

Water Quality Hazards

Prescribed fires have much less effect on water quality than wildfires. One way to lessen the impact that wildland fires have on water quality is to increase the use of prescribed fires.

Some of the declines in water quality associated with forest fires result from natural phenomena while others result from human actions. To some degree these consequences cannot be prevented since wildfires are unpredictable and difficult to control. But, prescribed burning, which reduces the risks of catastrophic wildfires, can help reduce damage to water quality.

Soil erosion, sedimentation, increases in nutrient loads, and increased turbidity are potential consequences of intense forest fires. These consequences may threaten human health by introducing bacteria, pathogens, and toxins into drinking water supplies.

Fire suppression and control techniques potentially damage water quality (Landsberg and Tiedemann 2000; Norris and Webb 1988). For instance, the use of fire retardants and chemical foams may cause the introduction of harmful chemicals (e.g., nitrates and ammonia). The construction of fire breaks or firelines may cause erosion and the runoff of nutrients into water supplies. Post-treatment techniques, such as the application of nitrate fertilizers to encourage the re-growth of vegetation, may increase the potential for human exposure to toxic substances. Humans may be exposed to the nitrates that are washed into drinking water supplies.

Water quality (along with visibility, odor, flora, fauna, wildlife, soils, and ecosystem integrity) is classified as an Air Quality Related Value (AQRV) in the Clean Air Act Amendment (Tonnassen 2000). The effects of forest fires on water quality vary due to differing characteristics of the particular fire and the environment in which it occurs. Slope, ground cover, precipitation, and temperature all influence the water quality changes that occur in burned areas. In addition, fire intensity and severity, and post-burn treatments affect water quality. Fire severity – as a measurement of the amount of fuels burned and nutrients released – is particularly influential on water quality changes. The potential for erosion rises in association with fire severity: more severe fires cause more dramatic changes in ground cover (Landsberg and Tiedemann 2000).

Erosion poses risks to human well-being. In some ecosystems, forest fires increase the frequency of flooding which is a natural hazard that can pose threats to human well-being, sometimes leading to premature death (Machlis 2002). The runoff of sediment and nutrients (e.g., nitrate and nitrite) that accompanies erosion causes declines in water quality.

Erosion: turbidity and sedimentation

Increases of sediment in drinking water poses risks to human health (Landsberg and Tiedemann 2000). Excessive amounts of sediment may threaten the operation of water treatment facilities. Turbidity in streamflow often increases after a forest fire. Turbidity poses indirect threats to human health by encouraging microbial production which increases the risks for infections in humans.

Changes in water chemistry

Water chemistry can be affected directly by input of nutrients and other substances in eroding sediment, and by the direct diffusion of biomass smoke into surface water. Thus, forest fires can contribute to eutrophication of water when additional nutrients are added, particularly nitrogen and phosphorus. Diffusion is a source of nitrogen in water supplies (Landsberg and Tiedemann 2000), as is runoff. Similarly, excess phosphorous partly results from the leaching of ashes that drop and dissolve directly in streamwater (Landsberg and Tiedemann 2000). It has been suggested that forest fires increase the concentration of dissolved salts in drinking water, but this has not been adequately demonstrated (Van Lear and Waldrop 1989). Mercury, a toxic metal that is a powerful neurotoxin (Tonnassen 2000), is sometimes present in forest fire smoke and may be deposited in water supplies. Human exposure to mercury can occur through ingestion of freshwater species and wildlife as well as through the inhalation of biomass smoke. Mercury concentrations may be particularly significant in the South. The Mercury Deposition Network (MDN) is a congregation of 30 sites throughout the United States that are monitored for annual mercury deposition and concentration in wetfall. The highest mercury concentrations in 1997 were measured in the Everglades National Park, Florida (27.2 g/m²). The third highest mercury concentrations were in Congaree Swamp National Monument, South Carolina (13.5 g/m²) (Tonnassen 2000).

Changes in the light environment

In some fire situations, biomass smoke and haze may decrease the amount of ultraviolet light (UV-B) that reaches surface water. The risk to human health occurs when a reduction in UV-B is sufficient enough to increase the growth of bacteria and pathogens in water supplies (Malilay 1999). In other situations, forest fires increase the exposure of surface waters to sunlight. Water temperatures may increase when fires burn off riparian vegetation exposing water sources to more direct sunlight. The erosion of nutrients into surface waters that can occur after fire may result in eutrophication which, when combined with increased sunlight and higher water temperature, can change the “color, smell, and taste of drinking water” and encourage the growth of microbes (Landsberg and Tiedemann 2000: 128).