How to Protect Yourself

Adapted by the Missoula City-County Health Department from “Wildfire Smoke, A Guide for Public Health Officials, 2002” which was written by Harriet Ammann, Washington Department of Health; Robert Blaisdell and Michael Lipsett, California Office of Environmental Health Hazard Assessment, Susan Lyon Stone, U.S. Environmental Protection Agency; and Shannon Therriault, Missoula City-County Health Department. The entire document can be viewed and printed by clicking here.

Staying Indoors

The most common advice given during a smoke pollution episode is to stay indoors. How well this works depends entirely on how clean the indoor air is. This strategy can usually provide some protection, especially in a tightly closed, air-conditioned house in which the air-conditioner can be set to re-circulate air instead of bringing in outdoor air. Staying inside with the doors and windows closed can usually reduce exposure to ambient air pollution by about a third. In homes without air conditioning, indoor concentrations of fine particles can approach 70 to 100 percent of the outdoor concentrations. In very leaky homes and buildings, staying inside with doors and windows closed may offer little protection. If doors and windows are left open, pollution levels indoors and outdoors will be about the same.

The possibility of heat stress is also an important drawback of staying inside during smoke events. The fire season typically extends from mid-summer through the early fall, when temperatures are often high. For those who depend on open windows and doors for cooling, keeping windows and doors closed can be problematic. Older individuals and others in frail health run the risk of heat exhaustion or heat stroke. If outdoor temperatures are very high, and you don’t have air conditioning, you could stay with friends or family who do, spend time at places with air conditioning (like the mall), or leave the area until the heavy smoke subsides.

Smoke events are usually interrupted by periods of relatively clean air. When air quality improves, even temporarily, you should “air out” your home to reduce indoor air pollution. During those times of less smoke, you can also clean your home, including mopping, dusting, and vacuuming, to help rid your home of particles that may have settled and can get stirred back up into the air.

Reduced Activity

Reducing physical activity is an important and effective strategy to lower the dose of inhaled air pollutants and minimize health risks during a smoke event. During exercise, people can increase their air intake as much as 10 to 20 times their resting level. Increased breathing rates bring more pollution deep into the lungs. Furthermore, while exercising, people tend to breathe through their mouths, bypassing the natural filtering ability of the nasal passages, again delivering more pollution to the lungs. They also tend to breathe more deeply, modifying the normal patterns of particle deposition in the lungs.

Reduce Other Sources of Indoor Air Pollution

Many indoor sources of air pollution can give off large amounts of the same pollutants present in forest fire smoke. Indoor sources such as burning cigarettes, gas, propane and wood-burning stoves and furnaces, and activities such as cooking, burning candles and incense, and vacuuming can greatly increase the particle levels in a home and should be avoided when high levels of wildfire smoke is present. For instance, in a room of 125 square feet, it takes only 10 minutes for the side-stream smoke of 4 cigarettes to create levels of particles in the hazardous ranges (644 micrograms of particles per cubic meter of air or mg/m3). Besides cigarette smoke, combustion sources that do not properly vent to the outdoors (including “room-vented” or “vent-free” appliances) contribute most to indoor pollutant levels and are of greatest concern. Reducing indoor air pollutant emissions during smoke events may reduce indoor particle levels by one quarter to one third or more. These reductions can help compensate for the increased particle loading from the outdoor air.

Air Conditioners

Little is known about the impact of using various types of air conditioners and air filters on indoor air pollutant concentrations. The conventional wisdom is that air conditioners reduce the amount of outdoor particles infiltrating indoors because air-conditioned homes usually have lower air exchange rates than homes that use open windows for ventilation. However, some air conditioners have both “outdoor air” and “re-circulate” settings; these air-conditioners need to be set on “re-circulate”. If possible, you should replace the air-conditioner filter with a pleated medium efficiency filter. However, caution must be taken to assure that the system is able to handle the possible increased airflow resistance. Some air conditioners may also be fitted with filters. The more useful are HEPA (High Efficiency Particulate Arrestor) filters, which can capture most of the tiny particles associated with smoke and can further reduce the amount of outside air pollution that gets indoors.

Room air cleaners

Some air cleaners can be effective at reducing indoor particle levels, provided the air cleaner is adequately matched to the indoor environment in which it is placed. Unfortunately, air cleaners tend to be expensive: they are available as either portable units designed to clean the air in a single room ($50 - $300) or as larger central air cleaners intended to clean the whole house ($300 - $1000+). In addition, most air cleaners are not effective at removing gases and odors. The two basic types for particle removal include:

  • Mechanical air cleaners, which contain a fiber or fabric filter. The filters need to be sealed tightly in their holders, and cleaned or replaced regularly.
  • Electronic air cleaners, such as electrostatic precipitators (ESPs) and ionizers. ESPs use a small electrical charge to collect particles from air pulled through the device. Ionizers, or negative ion generators, cause particles to stick to materials (such as carpet and walls) near the device. Electronic air cleaners usually produce small amounts of ozone (a respiratory irritant that can damage lungs) as a byproduct.

The effectiveness of an air cleaner is usually reported in terms of efficiency, which can be misleading, as it only tells half of the story. The other important factor is airflow. Together, these two factors equal the Clean Air Delivery Rate (CADR), which is a better measure of how a device will actually perform. For example, 99.99 percent efficiency sounds great, but if the air exchange rate is only 20 cubic feet per minute (cfm), you would be better off at 90 percent efficiency with 100 cfm air exchange rate (CADR: 20 vs 90 cfm).

Room air cleaners should be sized to filter at least two or three times the room volume per hour. Most portable units will state on the package the unit’s airflow rate, the room size it is suitable for, its particle removal efficiency and perhaps its CADR. Central system air units should handle at least 0.5 air changes per hour, the air exchange rate necessary to reasonably ventilate a house continuously under most conditions.

High and medium efficiency media filters and electrostatic precipitators can be added to central air conditioning systems to keep pollution in indoor air within acceptable levels during a prolonged smoke event. However, these filters create greater air resistance in the air conditioning system, and may require modifications to the system. In addition, electronic air-cleaners can increase indoor levels of ozone, as noted above.

Devices that remove gases and odors are relatively costly, both to purchase and maintain. They force air through materials such as activated charcoal or alumina coated with potassium permanganate. However, the filtering medium can become quickly overloaded and may need to be replaced often.

Ozone Generators – A Poor Choice

Some devices, known as ozone generators, personal ozone devices, “energized oxygen” generators, and “pure air” generators, are sold as air cleaners, but the position of public health agencies, including the US Environmental Protection Agency, is that they do more harm than good. These devices are designed to produce ozone gas to react with pollutants in the air. Ozone is composed of three atoms of oxygen. The third atom can detach from the molecule and reattach to molecules of other substances, altering their chemical composition. It is this ability to react with other substances that forms the basis of the manufacturers’ claims.

Ozone, whether in its pure form or mixed with other chemicals, can be harmful to health. When inhaled, ozone can damage the lungs. Relatively low amounts of ozone can irritate the airways, cause coughing, chest pain and tightness, and shortness of breath. It can also worsen chronic respiratory diseases such as asthma, as well as compromise the body’s ability to fight respiratory infections. As a result, using an ozone generator during a smoke event may actually increase the adverse health effects from the smoke. In addition, ozone does not remove particles from the air, and would therefore not be effective during smoke events. Some ozone generators include an ion generator to remove particles, but it would be far safer to buy the ionizer by itself.

For more information about residential air cleaners:

For more information about ozone generators marketed as air cleaners:


Humidifiers are not air cleaners, and will not significantly reduce the amount of particles in the air during a smoke event. Nor will they remove gases like carbon monoxide. However, humidifiers may slightly reduce pollutants through condensation, absorption and other mechanisms. In Missoula, one possible benefit of running a humidifier during a smoke event might be to help the mucus membranes remain comfortably moist, which may reduce eye and airway irritation. However, the usefulness of humidification during a smoke event has not been studied.

Inside Vehicles

Individuals can reduce the amount of smoke in their vehicles by keeping the windows and vents closed. However, in hot weather a car’s interior can heat up very quickly to temperatures that far exceed those outdoors, and heat-related stress can result. Children and pets should never be left unattended in a vehicle with the windows closed. In hot weather, children and pets should not be left unattended in a vehicle with the windows even partially open. The car’s ventilation system typically removes a small portion of the particles coming in from outside. Most vehicles can re-circulate the inside air, which will help keep the particle levels lower. Drivers should check the owner’s manual and assure that the system is set correctly to minimize entry of outdoor smoke and particles.


In general, wearing a mask is not an effective strategy to reduce your exposure to wildfire smoke. In order for a mask to provide protection, it must be able to filter very small particles (around 0.3 to 0.1 micrometer) and it must fit well, providing an airtight seal around the wearer’s mouth and nose. Commonly available paper dust masks, which are designed to filter out larger particles, such as dust created by sanding, typically offer little protection. The same is true for bandanas (wet or dry) and tissues held over the mouth and nose. Surgical masks that trap smaller particles are also available, but these masks are designed to filter air coming out of the wearer’s mouth, and do not provide a good seal to prevent inhalation of small particles or combustion gases. As a result, these tend to be no better than dust masks. In fact, masks may actually be detrimental, giving the wearers a false sense of security, which may encourage increased physical activity and time spent outdoors, resulting in increased exposures.

There are several additional drawbacks to recommending widespread mask use in an area affected by wildfire smoke. Most people won’t use the masks correctly and won’t understand the importance of having an airtight seal. For instance, it is impossible to get a good seal on individuals with beards or mustaches. In addition, such masks aren’t designed for use by the general public (including children). As a result, masks will provide little, if any, protection.

Masks are uncomfortable (they are more comfortable when they are leaky – but then they do not provide protection). They increase resistance to airflow. This may make breathing more difficult and lead to physiological stress, such as increased respiratory and heart rates. Masks can also contribute to heat stress. Because of this, mask use by those with cardiac and respiratory diseases can be dangerous, and should only be done under a doctor’s supervision. Even healthy adults may find that the increased effort required for breathing makes it uncomfortable to wear a mask for more than short periods of time. Breathing resistance increases with respirator efficiency. A final problem with most masks is that they do not filter out harmful irritant gases, such as acrolein or formaldehyde, or other toxic gases, such as carbon monoxide.

There are, however, some situations in which mask use can be beneficial. For outdoor workers, or others who will be outside regardless of the smoke, properly fitted masks can afford some protection. In cases where people are generally staying indoors, wearing a mask to go outside briefly might be useful. Masks can also be used in conjunction with other methods of exposure reduction, including staying indoors, reducing activity, and using HEPA air cleaners to reduce overall smoke exposure.

Some flexible masks (technically called respirators, but they look more like paper masks) are good enough to filter out 95 percent of the particulate matter that is 0.3 micrometers and larger. Smoke particles average about 0.3 micrometers in diameter, so these masks can filter out a significant portion of the smoke if they are properly fitted to the wearer’s face. These masks, which may include an exhalation valve, do not require cartridge filters. They are marked with one of the following: “R95”, “N95” or “P95.” These are typically sold at medical supply and home improvement stores, and tend to be more expensive than ordinary dust masks. Soft masks with higher ratings (R, N or P99 and R, N, or P100) are also available and will filter out even more particles. Again, if a respirator does not provide a tight seal, it will not be effective.

Individuals who want added protection can use tight-fitting respirators that require cartridge filters. Respirators with purple HEPA filters offer the highest protection, but may be less comfortable and slightly more expensive than the flexible masks. You can get respirator cartridges with a combination N95 or N99 filter and an organic vapor backup. This combination can help reduce exposure to some gases, such as benzene and irritant aldehydes, as well as particles. Again, unless there is an airtight seal over the wearer’s mouth and nose, such respirators will provide little protection.

Leaving the Area/Evacuations

One of the most effective, but for many the least practical, solution is to leave the area until smoke subsides. However, it is often difficult to predict the duration, intensity and direction of smoke, creating uncertainty about how necessary or effective this course of action would be.

The smoke changes quickly, with heavy smoke interspersed with times of relatively clean air. By the time you pack up to leave, the air may clear out. By the time you unpack, the valleys might be choking once again with smoke. Since smoke can affect a whole region, especially with as many fires as we have this year, finding a place to get out of the smoke can be difficult.