What's really in the air inside your home?

The EPA estimates people spend about 90% of their time indoors, most of it at home. The air in there isn't sealed off from the outside, and it isn't always cleaner. In the EPA's field studies, some common household chemicals ran two to five times higher indoors than outdoors.

The good news is that most of what shapes your indoor air comes down to a few things you can see and change: what's being made inside, how your house trades air with the outdoors, and where moisture collects. This guide walks through each one, along with the fixes we see make the biggest difference.

Where indoor air comes from

The EPA boils indoor air problems down to two things working together: sources that give off gases or particles, and too little ventilation to carry them away. Some sources come from outside, like smoke and traffic. Others start inside, with cooking, cleaners, paint, new furniture and the ground under the house.

Your house trades air with the outdoors all the time, even with every window shut. Air slips in around windows and doors, and wherever walls meet floors and ceilings. How much gets through depends on how tight the house is and how you run it. One large study found the average home let in about 60% of the fine particles outside. Newer, well-sealed homes with air conditioning kept much more of it out.

Smoke days and smoggy days

If you live in the West, you know the drill. When wildfire smoke rolls in, some of it gets inside. In one field study of homes during real smoke events, indoor fine particles still ran at about half the outdoor level with the windows closed. Closing up helps, but filtering does most of the work: portable HEPA cleaners in the same study cut indoor smoke particles by roughly half to three quarters.

What the EPA recommends on smoke days:

• Set your HVAC to recirculate and switch the fan from Auto to On, so it filters all day. Use a MERV 13 filter, or the highest rating your system can handle. Ask an HVAC tech if you're not sure.
• Set up one room, often a bedroom, as a clean room. Keep it closed, run a portable HEPA cleaner in it, and skip frying, candles and vacuuming without a HEPA filter.
• No air cleaner? A MERV 13 filter taped to a box fan is a real stopgap. Use a fan made in 2012 or later with a UL or ETL mark, don't leave it running while you sleep, and change the filter often.

To see what the air is doing near you, check the AirNow Fire and Smoke Map at fire.airnow.gov.

Smog works the same way on a smaller scale. Ground-level ozone builds on hot, sunny afternoons and drifts in through the same gaps. It's the one time fresh air isn't the answer: in one building study, opening the windows on a high-ozone day let in about five times as much ozone as keeping them shut. On bad-air days, keep the windows closed and let the filters work.

Ventilation: the everyday fix

The EPA ranks the ways to improve indoor air in order: remove the source first, then ventilate, then clean the air. Ventilation dilutes a problem more than it removes one, but it's the lever you can pull every day.

Run the kitchen fan when you cook and the bathroom fan when you shower, and make sure both vent outside. Open the windows while you paint, sand or use strong cleaners, as long as the air outside is good.

Newer homes are built tighter to save energy, which also means they hold on to whatever is made inside. The Department of Energy's rule of thumb is "build tight, ventilate right": a tight home needs whole-house mechanical ventilation. Heat- and energy-recovery ventilators (HRVs and ERVs) bring in fresh air while keeping most of the heating or cooling you've already paid for.

Filters: what the numbers mean

Your furnace filter's MERV rating runs from 1 to 16. The EPA recommends at least MERV 13 for homes, which has to catch at least half of the finest particles it's tested on. Higher ratings also slow the airflow, so check what your system can handle before you buy.

The most common filter problem we see is simpler than any of that: a filter that hasn't been changed in a long time, or one that's the wrong size and lets air slip around the edges. A new filter that fits is the cheapest upgrade in the house.

For a portable air cleaner, look at the Clean Air Delivery Rate (CADR). The EPA's rough guide is a CADR of about 65 for a 100-square-foot room, up to about 390 for 600 square feet. The AHAM Verifide seal means that number was independently tested, and a true HEPA filter catches at least 99.97% of particles at the size that's hardest to catch.

Two limits worth knowing. Particle filters don't remove gases or odors, so they won't clear cooking fumes or a musty smell. And skip ozone generators sold as air purifiers: the EPA notes that no federal agency has approved them for use in occupied rooms.

The parts of the house you don't see

Warm air rises and leaks out the top of a house, which pulls air in at the bottom. Building scientists call it the stack effect, and it means the crawl space, the basement and the attic all feed the air in your rooms.

The crawl space

Crawl spaces with standing water, or with no vapor barrier at all, are some of the most common problems we find. For years the standard advice was to vent crawl spaces to the outside, but in humid weather that outside air often brings in more moisture than it carries out. The Department of Energy now favors sealed, conditioned crawl spaces, which stay noticeably drier.

Sealing a crawl space also changes how soil gases like radon move under the house, which is why DOE guidance adds a radon check once the work is done.

The basement

Water is the basement's main problem, and most of it starts outside. One inch of rain puts about 1,250 gallons on the roof of a 2,000-square-foot house, and downspouts that empty right at the foundation send much of it straight toward the walls. We see that one a lot. Extend downspouts at least 4 feet from the house, and grade the soil so it slopes away, about an inch per foot for the first 6 feet.

Inside, the EPA's targets are simple: keep humidity below 60% (30% to 50% is ideal), and dry anything that gets wet within 24 to 48 hours.

The attic

Moisture is the attic's biggest problem too, and a lot of it is piped in by accident. We regularly find bathroom fans and dryers venting into the attic, and dryers venting into the garage or the crawl space. All of these should run all the way outside. The Department of Energy calls venting into the attic a major source of moisture.

Powered attic fans usually make things worse: in a house that isn't well sealed, they pull indoor air up through gaps in the ceiling. The fixes that work are unglamorous. Seal the gaps in the ceiling, add insulation, and keep the soffit vents clear. If your ducts run through the attic, seal the joints with mastic or foil tape. ENERGY STAR estimates a typical duct system loses 20% to 30% of its air through leaks.

Moisture first, then mold

Mold follows water. The EPA puts it in one line: moisture control is the key to mold control. When the humidity, the leaks and the vents are handled, mold has little to grow on.

Winter is when it shows. Condensation on the windows and damp, dark corners on cold walls mean the indoor air is holding more moisture than the house can handle, and we see both often. Run the bathroom and kitchen fans longer, make sure the dryer vents outside, and bring the humidity down.

For a small patch of mold, under about 10 square feet, the EPA says a homeowner can usually clean it up. Anything bigger is a job for a pro. If you want to know what a spot is before you decide who to call, a swab or tape lift will tell you.

Where to start

Most indoor air problems trace back to a few places: what's made inside, how the house breathes, and where water collects. Some of them you can spot yourself, like a fan that vents into the attic or a downspout at the foundation. Others you can't see, and that's where testing helps.

Not sure which test fits your home? Answer a few quick questions and we'll point you to one: Find your test.

Sources

• U.S. EPA, The Inside Story: A Guide to Indoor Air Quality
• U.S. EPA, Wildfires and Indoor Air Quality
• U.S. EPA, Guide to Air Cleaners in the Home
• U.S. EPA, A Brief Guide to Mold, Moisture and Your Home
• U.S. Department of Energy, Building America guidance on ventilation, crawl spaces and attics
• ENERGY STAR, Seal and Insulate
• AirNow, AQI Basics and the Fire and Smoke Map
• University of Minnesota Extension, Moisture in Basements