Increased outdoor temperatures and heat waves are expected to lead to increased wildfires. Data suggest a large increase since 1983 in area burned per year in the U.S. [23], although the large year-to-year variability make conclusions difficult. Climate change is also projected to increase the number and severity of droughts in some regions of the world, also contributing to increased wildfires. For example, Spracklen, Mickley [38] have estimated that, by 2050, climate change will cause a 54% increase in the average area burned in the western U.S.

Wildfires can cause temporary large increases in outdoor airborne particles, and substantial increases in gaseous air pollutants such as carbon monoxide, nitrogen dioxide, formaldehyde, and acetaldehyde [39-42]. Large wildfires can increase air pollution over thousands of square kilometers [or thousands of square miles] [11, 40, 43]. Calculations based on model projections [38] indicate that climate-change-driven wildfires and changes in outdoor particle transport will increase summertime mean outdoor air levels of fine particles in the western U.S. by thirty to forty percent. Percentage increases in fine particles in urban areas that have higher current particle concentrations are likely to be smaller. Analyses of particle data indicate a several fold increase in outdoor airborne particles during a wild fire that occurred in Southern California in 2003 [40]. Researchers found that population-weighted concentrations of particles less than 2.5 micrometers in diameter (PM2.5) were 90 micrograms per cubic meter under heavy smoke conditions and 75 micrograms per cubic meter under light smoke conditions, which compare to 20 micrograms per cubic meter during the non-fire periods. For particles less than 10 micrometers in diameter (PM10), population-weighted concentrations were 190 micrograms per cubic meter under heavy smoke conditions and 125 micrograms per cubic meter under light smoke conditions, which compare to 40 micrograms per cubic meter during the non-fire periods.

During an extreme long-term fire in Indonesia, highly affected areas had more than 1000 micrograms per cubic meter of PM10 for several days and long periods with more than 150 micrograms per cubic meter [44]. Concentrations of PM10 as high as 1860 micrograms per cubic meter were reported [45]. U.S. EPA National Ambient Air Quality Standards specify that concentrations of PM10 particle pollution should not exceed 150 micrograms per cubic meter, on average, more than once per year.

Several, but not all, studies have documented increases in adverse health effects in populations exposed to pollutants from wildfires. Health effects assessed in these studies have included hospital admissions for various causes, mortality, respiratory symptoms (such as cough and wheeze), eye and nose symptoms, and respiratory infections (colds, bronchitis, and pneumonia) [39, 41, 43, 44, 46]. The elderly, infants, and those with preexisting respiratory diseases such as asthma and chronic obstructive pulmonary disease may be most susceptible. In general, the studies have compared the prevalence of adverse health effects in a defined population during periods with and without exposures to pollutants from wildfires. The concentrations of air pollutants in populations exposed to wildfires, and the periods of time over which pollutant concentrations are elevated, vary widely among wildfire events. Also, the baseline level of health and access to health care of exposed populations vary widely. Consequently, one would not expect different studies to detect the same size increase in adverse health effects.

Table 2 summarizes several studies of the health effects of pollution from wildfires. The first study of a Southern California population reports several statistically significant increases in in hospital admissions related to respiratory health effects, ranging from 3% to 10% per each 10 microgram per cubic meter increase in ambient PM2.5. The second study of high school students during the same wildfire event reported increases in irritation and respiratory symptoms, medication use, and doctor visits that usually were between 30% and 300%. In most cases the increases were statistically significant. The third study, of two communities in British Columbia, reports statistically significant 46% to 78% increases in visits to physicians for respiratory health effects in the more highly affected community, but no statistically significant increases in the second community. The fourth study found no statistically significant increase in mortality in Denver associated with short-term exposures to pollutants from wildfires. The last two studies address the health effects of a very large multiple-month wildfire in Indonesia. A study of an affected Malaysian community reports a statistically significant 70% increase in non-traumatic deaths among residents with an age of 64 to 74, and a non-significant 19% increase for all ages. The final study analyzed census data and suggested that wildfire pollution is responsible for 15,600 child, infant, and fetal deaths, corresponding to a 1.2% decrease in survival of those born or expected to be born during the period of exposure.

Table 2. Reported health effects of wildfires (from [1]).

Reference Study Population Particle Levels µg/m3 Duration of Exposure Increases in Health Effects
(bold if statistically significant)


Residents of five Southern California counties

PM2.5* increases compared to non-fire periods:

55 (light smoke areas)
70 (heavy smoke areas)
240 (peak 24-h average)

1.5 month

Asthma admissions
26% and 34% for 55 and 70 µg/m3 PM2.5 (all ages)
10.1% per 10 µg/m3 PM2.5 (age ≥ 65)
8.3% per 10 µg/m3 PM2.5 (age ≤ 4)
No effect (age 5 - 8)
Acute bronchitis admissions
9.6% per 10 µg/m3 PM2.5 (all ages)
COPD*** admissions
6.8% per 10 µg/m3 PM2.5 (age 20-64)
Pneumonia admissions
2.8% per 10 µg/m3 PM2.5 (all ages)
Cardiovascular admissions (all ages)
No significant effects per 10 µg/m3 PM2.5 (all ages)


873 high school students and 5551 elementary grade students from 16 towns in Southern California

PM10** Five-day mean levels in 16 towns ranged from 30 to 252

~ 1.5 month

Eye, nose and throat symptoms, cough, bronchitis, cold, wheezing, asthma attacks, medication use, and physician visits increased generally by 30% to 300% most increases are statistically significant


Kelowna and Kamloops communities in British Columbia

PM2.5 Peaks:

200 in Kelowna
140 in Kamloops

5 weeks

In Kelowna 46% to 78% increase in physician visits for respiratory diseases (statistically significant during 3 weeks)

In Kamloops no statistically significant increases in physician visits


Residents of Denver metropolitan area

PM2.5 Peaks of 200 for 4-5 hours on each of two days

2 days

No clear statistically significant increases in mortality


Residents of Kuala Lampur Malaysia

Variable with PM10 above 166 on 20 days and above 245 on 8 days

Episodic over several months

Increase in non-traumatic death with PM10 > 210

70% (age 65-74)

19% (all ages)


Residents of Indonesia

Not available

~ 4 months

Census data indicate 15,600 child, infant, and fetal deaths, a 1.2% decrease in survival of those born or expected to be born during period of exposure

*PM2.5 = particles less than 2.5 micrometers in diameter

**PM10 = particles less than 10 micrometers in diameter

*** COPD = chronic obstructive pulmonary disease

When outdoor air particle concentrations increase, indoor air concentrations of particles also increase, particularly in homes because they usually have low efficiency particle filtration systems or no particle filtration. Based on analyses for a set of Boston-area homes [49], increases in indoor particle concentrations during wildfires will be between 49% and 76% of the increases in outdoor air particles, with the range dependent on particle size for particles between 0.25 and 5 micrometer in diameter. Modeling by Riley et al. [50] indicates that increases in indoor particle concentrations will be 33% to 44% of increases in in outdoor particle concentrations for California homes with central air heating and cooling systems when windows are closed. The percentages are 64% to 80% for homes with typical air infiltration and no central air and 83% to 95% for homes with open windows. Their modeling suggest 53% to 72% increases in particles in offices with low efficiency particle filters and 13% to 18% increases in offices with high efficiency particle filters, relative to the increases in particles outdoors. The lower percentages and upper percentages in each range apply for PM10 and PM2.5, respectively.

Because people in the U.S. and many other developed countries are indoors approximately 90% of the time, and may be indoors even more when outdoor air is affected by wildfires, increases in exposures to particles from wildfires attributable to climate change will occur primarily indoors. Thus, the adverse health effects expected from increased wildfires will substantially be the consequence of exposures to particles that penetrate to and persist indoors. Based on the discussion in the prior paragraph, in developed countries, increases in indoor concentrations of particles from wildfires are roughly 50% of the increases in outdoor concentrations. Combining this percentage with the 90% of the time that people in the U.S. are indoors, one can estimate that roughly 80% of total exposure to particles from wildfires will occur indoors. The most affected population (infants, the elderly, and those with respiratory diseases) may be indoors more than 90% of the time, increasing the significance of indoor exposures. If adverse health effects scale directly with the total increase in particle exposure, one could estimate that roughly 80% of the adverse health effects of wildfires in the U.S. are attributable to indoor exposures. However, it is possible that health effects are not linearly proportional to the total increase of particle exposure, causing indoor exposures to account for less than 80% of total adverse health effects of particles from wildfires.

Based on the information in the preceding paragraph, it is clear that the health effects of indoor exposures to particles from wildfires are important, and are likely larger than the effects of exposures to these particles that occur when people are outdoors. There are associated options to reduce the health effects of pollutants from wildfires. These options include doing the following when air is polluted by emissions from wildfires: 1) spending more time indoors; 2) keeping windows and doors closed; and 3) operating particle filtration systems. The particle filtration systems referenced in option 3 could be those installed in forced-air heating and cooling systems, with fans run continuously when there is pollution from wildfires. To be highly effective for particles from wildfires, the filters installed in these systems should have a higher particle removal efficiency than is typical of current practice in U.S. homes [51]. Alternately, portable fan-filter systems (particle air cleaners) could be operated during wildfires. A side advantage is that routine use of either of these particle filter systems would also be expected to yield health benefits from reduced exposures to everyday sources of particles [52].

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