Wildfires

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 makes 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 et al. [38] have estimated that, by 2050, climate change will cause a 54% increase in the average area burned in the western U.S. In a review of the expected health impacts of climate change, the U.S. Global Climate Research Program [39] identified increased health impacts from wildfires as one of the key findings.

Outdoor Air Pollutants Increase During Wildfires

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 [40-43]. Large wildfires can increase air pollution over thousands of square kilometers [or thousands of square miles] [11, 41, 44]. 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 [41]. 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. Liu et al. [45] plotted data from several studies during which PM2.5 levels increased by approximately 30 to 100 micrograms per cubic meter during the wildfires.

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 [46]. Concentrations of PM10 as high as 1860 micrograms per cubic meter were reported [47]. 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.

Health Effects from Exposure to Wildfire Smoke

Liu et al. [45] identified 61 studies of the health effects of wildfire smoke exposure. 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, respiratory infections (colds, bronchitis, and pneumonia), measures of respiratory system function, and measures of inflammation. In 41 of 45 studies that examined effects of wildfire smoke on respiratory morbidity, there was a statistically significant increase in morbidity. Six of 14 studies reported statistically significant increases in cardiovascular morbidity with wildfire smoke exposure and nine of 13 studies reported statistically significant increases in mortality. The duration of the period of wildfire smoke exposure and the concentrations of smoke pollutants varied highly among the reviewed studies. Also, the baseline level of health and access to health care of exposed populations vary widely. Consequently, it is not surprising that the size of the increased risks of adverse health effects varied widely. For example, the increases in contacts with hospitals or clinics (often hospital admissions) during wildfires ranged from nil to well over 100% and increases in mortality ranged from less than 1% to approximately 50%. Johnston et al. [48] estimated that landscape fires, consisting of wildfires and prescribed burns, globally cause 339,000 premature deaths per year, with a range of 260,000 to 600,000 deaths. In general, the elderly and young children were found to more often experience adverse health effects from wildfire smoke exposure. People with preexisting respiratory diseases such as asthma and chronic obstructive pulmonary disease may be most susceptible.

Effects of Wildfires on Indoor Particle Levels and Exposures

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.

Options to Reduce Health Effects of Air Pollutants from Wildfires

There are associated options to reduce the health effects of pollutants from wildfires. These options include doing the following when outdoor air is polluted by emissions from wildfires: 1) spending more time indoors; 2) keeping windows and doors closed; and 3) operating indoor particle filtration systems. Note that when sheltering indoors with windows and doors closed, it is important to reduce other indoor sources of particulate matter such as smoking, cooking, burning candles or incense, and unvented combustion equipment. Also, it may be uncomfortable and pose health risks to remain indoors with doors and windows closed in homes without air conditioning, as wildfires often occur during hot summer and fall weather, and it may be necessary to seek protection via other means.

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].

Two identified papers report experimental evaluations of air cleaner operation in homes during wildfire periods. In one study [53], operation of portable air cleaners in 17 homes reduced concentrations of PM2.5 by 65% ±35%. In another study with air cleaner operation in five homes [54], the estimated reduction of PM2.5 was 63% to 88%. A study completed under this IAQ Scientific Findings Resource bank project [55] used mathematical models together with measured data to evaluate the health and economic benefits and costs of employing various filtration systems if used in homes during a prior wildfire in Southern California. As shown in Table 2 based on this study plus supplemental calculations, the six interventions implemented in all wildfire-affected houses were projected to prevent 11% to 76% of the hospital admissions and 7% to 47% of the deaths attributable to wildfire particles. The estimated economic value of the prevented deaths exceeded or far exceeded intervention costs for interventions that did not use portable air cleaners. For the interventions with portable air cleaner use, mortality-related economic benefits exceeded intervention costs as long as the cost of the air cleaners, which have a multi-year life, were not attributed to the short wildfire period. Cost effectiveness was improved by intervening only in the homes of the elderly who experience most of the health effects of particles from wildfires and who were assumed to stay indoors at home during the entire wildfire period. For this elderly population, the predicted prevented hospital admissions sometimes exceeded the increase in hospital admissions attributable to the wildfire because the interventions reduced particle exposures to below the exposures typical of periods without a wildfire. Similar improvements in cost effectiveness would also be expected from targeting interventions only at the homes or other high-risk groups or at the homes of other groups expected to stay indoors at home during the entire wildfire period.

Table 2. Summary of health benefits and costs of interventions that reduce indoor exposure to PM2.5 during wildfires.

Baseline or Intervention code Reference Condition Conditions Interventions in All Homes Interventions in Homes with Resident Age ≥ 65
Forced Air System Operation Efficiency of Filter in Forced Air System Continuously Operating Portable Air Cleaner* % Hospital Admissions Avoided % Premature Deaths Avoided % Hospital Admissions Avoided % Premature Deaths Avoided
B1 NA Intermittent Typical low No        
B2 NA No forced air NA No        
i1 B1 Continuous Typical low No 25 16 43 27
i2 B1 Continuous Upgraded to high No 48 30 81 50
i3 B1 Intermittent Upgraded to high No 11 7 20 12
i4 B1 Continuous Typical low

CADR / V = 1

CADR / V = 2

CADR / V = 3

53

65

71

32

40

44

88

108

119

55

67

73

i5 B1 Continuous Upgraded to high

CADR / V = 1

CADR / V = 2

CADR / V = 3

63

71

76

39

44

47

105

117

125

65

73

78

I6 B2 No forced air NA

CADR / V = 1

CADR / V = 2

CADR / V = 3

48

65

73

30

41

46

78

104

117

49

65

73

*CADR is the clean air delivery rate of the portable air cleaner and V is the house volume with CADR / V having units of 1 over hours.  For example, if CADR is 250 cubic feet per minute which converts to 15,000 cubic feet per hour and the house volume is 15,000 cubic feet, then CADR / V = 1 with units of 1 over hours.