Air isn't as light as it seems. It's pushing on your skin right now with up to 15 pounds of pressure per square inch, a weight so familiar you can't feel it. Your lungs feel it, though, especially when it's bogged down with toxins. And while we tend to think of air pollution as an outdoor threat, it can be even worse inside the buildings where we live and work.

The causes of indoor air pollution vary from region to region, house to house and even room to room. Contaminated air seeps in from outside, but it also wafts up from a smorgasbord of indoor sources like construction materials, consumer products, mold, insects and pets. Poor ventilation can let it accumulate to dangerous levels, a problem that often spikes in fall and winter as we seal up buildings to conserve heat.

If you're concerned about the air inside your home or office — two places where many people do the bulk of their breathing — you might want to pick it apart with indoor air-quality testing. To help you clear the air once and for all, here's a look at some of the most common indoor air pollutants, how to detect them and how to deal with them.

1. Combustion

gas flame, carbon monoxide, indoor air quality testingGases and particles from combustion are the leading sources of indoor air pollution worldwide. Household cookstoves alone kill about 4 million people every year, mostly in developing countries, but this category also includes heating stoves, fireplaces, furnaces, space heaters and tobacco smoke. The top pollutants released by combustion are carbon monoxide (CO), nitrogen dioxide (NO2) and particulate matter.

CO causes an array of symptoms — from headaches and nausea to confusion and unconsciousness — and kills about 500 people in the U.S. per year. NO2 irritates mucous membranes and causes shortness of breath, and long-term exposure to low levels may raise the risk of lung infections or emphysema. Airborne particulates can lodge in the lungs, potentially damaging tissue and even working their way into the bloodstream.

CO is colorless and odorless, so the best way to detect it is by installing CO alarms near bedrooms and fuel-burning appliances. Those appliances should also be inspected at least once a year by a qualified technician, as should chimneys, flues and air-handling systems. Other combustion products are easier to see and smell — NO2 is reddish-brown with an acrid odor, for example — but low levels can also be detected with certain instruments. And when using a stove, space heater or fireplace, ventilate with a fan or window.

2. Radon

porch - air quality testing under the homeAnother colorless and odorless gas, radon, is the No. 2 cause of lung cancer in the U.S., killing about 21,000 Americans every year. Nearly all soil contains low levels of decaying uranium, which emits radon, although certain regions have more than others. It normally dissipates harmlessly into outdoor air, but it can also flow into buildings through gaps in the foundation, eventually reaching unsafe levels in basements and lower floors.

While the EPA's radon zone map can hint at your general risk, air-quality testing is the only way to be sure. DIY radon test kits are available online and in home improvement stores, but the EPA suggests contacting your state radon office first, as some states offer free or discounted kits. And since careful steps must be taken to ensure an accurate reading, many people opt for a qualified radon inspector, especially when buying a home.

The average indoor radon level is 1.3 pico curies per liter (pCi/L), and the EPA recommends taking action if you detect a level of 4 pCi/L or higher. Radon remediation involves sealing off the building's interior from exposed soil, a complex task typically best left to professionals. The average cost is about $1,200, according to the EPA.

3. Asbestos

asbestos particles - air quality in old buildingsLike radon, asbestos occurs naturally in soil, posing little health risk until it gets indoors. While radon sneaks in, however, most asbestos is an invited guest that has overstayed its welcome. The heat-resistant mineral fiber has long been used as a building material and insulator, but its stock crashed in recent decades amid news that inhaling its fibers can cause lung cancer, mesothelioma and long-term lung scarring.

Most modern homes and offices now use alternative materials, but older buildings may still contain asbestos. Even then, the fibers only become airborne when they're disturbed, so the most practical solution is often to simply leave asbestos alone. That's not always an option, though — an aging home may need repairs in its asbestos-lined attic, for example, or squirrels may have kicked up the fibers while looking for a place to spend the winter.

Given the risks involved, DIY asbestos remediation is rarely a good idea. Even taking your own samples for testing isn't recommended. If you suspect a material contains asbestos, look for signs of damage without touching it, then contact a professional inspector to learn more. Federal law doesn't require accreditation for asbestos work in single-family homes, but some states and municipalities do. See this list of state asbestos contacts for help.

4. Volatile organic compounds

VOCs - indoor air quality testingVolatile organic compounds (VOCs) are in countless consumer products, from paint and glue to printers and shower curtains. They have low boiling points, causing them to "off-gas" lots of vapor even at room temperature. Some VOC vapors cause short-term health issues like headaches and nausea, often grouped together as "sick building syndrome." Others pose longer-term risks, from brain damage to cancer.

Average levels of certain VOCs are two to five times higher indoors than outdoors, according to the EPA, and they're prone to dramatic swings. During and for several hours after paint stripping, for example, indoor VOC levels can rise to 1,000 times the outdoor average. The best defense is to use VOC-containing products sparingly, and to keep their fumes from accumulating by using them outdoors or ventilating with fans and windows.

Testing for most VOCs isn't helpful, since no federal standards exist for non-industrial settings. A few VOCs warrant extra scrutiny, though, because of their links to cancer. One of the most infamous is formaldehyde, which wafts from pressed wood products, glues, textiles, gas stoves and tobacco smoke. The EPA suggests mitigation if indoor levels exceed 0.1 part per million. DIY test kits are available, but according to the University of Minnesota School of Public Health, "in cases where accuracy of results is important, only trained professionals should measure formaldehyde because of the difficulty of obtaining good data and interpreting results." Another carcinogenic VOC to avoid is benzene.

5. Mold and mildew

mold - indoor air quality testingFungi are notorious indoor air polluters, seizing on warm, humid conditions to colonize and contaminate. Outbreaks often begin in basements and bathrooms, but can quickly spread with enough moisture. Health effects vary by mold type and personal sensitivity; symptoms may include nasal stuffiness, wheezing and skin irritation. Studies have also linked indoor mold exposure to asthma development in children.

The best way to fight mold is to fight moisture. Keep the relative humidity indoors below 60 percent, and use a dehumidifier or fan to dry out the air if needed. Pockets of mold can be removed from hard surfaces by scrubbing with soap and water, a bleach solution or hydrogen peroxide, but check EPA guidelines for larger-scale cleanups.

Most molds cause human health problems, but our sensitivity varies and no federal standards exist, so air-quality testing is not usually advised as a first step. "You do not need to know the type of mold growing in your home, and CDC does not recommend or perform routine sampling for molds," the CDC says. "The best practice is to remove the mold and work to prevent future growth." Certain circumstances may call for more specific information, however, and most air-testing companies do offer mold inspection.

6. Dust, dander and droppings

rodent droppings - indoor air quality testingMold isn't the only biological polluter of indoor air. Many buildings are plagued by dust mites and cockroaches, two very different arthropods that both leave a trail of allergenic feces and body parts. Fumes from rodent urine and droppings can also cause breathing problems, as can pet dander and airborne proteins from cat saliva. On top of that, indoor air may be invaded by pollen and bacteria from outside.

These contaminants often trigger allergic reactions and asthma, and symptoms can grow worse with chronic exposure. Children, elderly people and people with other breathing issues are especially at risk from biological agents in confined areas, the EPA warns.

Testing can sniff out some biological pollutants, but as with mold, it may be easier to use visual clues. Regular sightings of roaches, rats or their droppings point to an infestation, in which case pest control is likely the best way to clear the air. Dust mites aren't visible to the naked eye, but we can see piles of their namesake food — and cleaning up dust may also alleviate allergies from pet dander. Beyond good housekeeping, ventilation can help keep unavoidable allergens from reaching high concentrations.

7. Pesticides

pesticidesEven though rodents, insects and other pests are a common source of indoor air pollution, eradicating them with poisons can raise the risk of trading one problem for another. Pesticides are inherently toxic, the EPA notes, often featuring organic compounds that add to existing levels of airborne VOCs. Health effects vary depending on the chemical and dosage, but symptoms of pesticide exposure range from headaches and nausea to long-term brain damage and increased risk of cancer.

Three-quarters of U.S. households have used at least one pesticide indoors during the past year, according to the EPA, mostly insecticides and disinfectants. These indoor chemicals account for up to 80 percent of most people's total exposure to pesticides.

Using non-chemical alternatives may be more practical than testing air for pesticides. Still, indoor air-quality tests have shown some homes' pesticide levels are too high to be explained by recent pesticide use alone, pointing to other sources like improperly stored pesticide containers or residues tracked inside from treated lawns. If you use chemical pesticides indoors, follow instructions on the label and ventilate thoroughly. Many carcinogenic pesticides like chlordane are now banned in the U.S., but be wary of mothballs that contain paradichlorobenzene, which has caused cancer in lab animals.

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For more information about indoor air quality and testing tactics, check out the EPA's introduction to indoor air quality and the related links below from MNN.

Related indoor air stories on MNN:

Click for photo credits

Gas stove: Damien Meyer/AFP/Getty Images

Porch and crawlspace: Ivy Dawned/Flickr

Asbestos: Charly Triballeau/AFP/Getty Images

Paint buckets: Erich Ferdinand/Flickr

Moldy wall: Matti Mattila/Flickr

Rat in a pipe: AFP/Getty Images

Insecticide pump: Wikimedia Commons