Cleaner Indoor Air, Starting at the Source

Most people spend the large majority of their time indoors, and the US Environmental Protection Agency has noted that levels of some pollutants indoors can be two to five times higher than typical outdoor levels. The good news is that indoor air is also more within your control. The most effective approach, used by building scientists, follows three steps in order: remove or reduce the source, ventilate, then filter. Looking at the main pollutants one by one shows which step matters most for each.

Fine particles

Where they come from: cooking, especially frying and searing, toasters, candles, incense, fireplaces and wood stoves, smoking, and outdoor sources such as traffic and wildfire smoke that drift inside.

Why they matter: fine particles known as PM2.5 can penetrate deep into the lungs and are linked with heart and lung disease.

What helps: use a range hood that vents outside every time you cook, and use the back burners, which the hood captures better. Avoid smoking indoors, and limit candles and incense. A portable air cleaner with a true HEPA filter, or a central heating and cooling system with a MERV 13 filter or higher, removes a large share of fine particles. During wildfire smoke events, keep windows closed and run filtration continuously.

Gases from combustion

Nitrogen dioxide comes mainly from gas stoves and unvented gas heaters, and can irritate airways, especially in people with asthma. Studies have linked gas stove use to higher rates of childhood asthma symptoms. Ventilating while cooking, using a range hood that vents outdoors, and opening a window help, as do induction or electric cooktops.

Carbon monoxide is a colorless, odorless gas from fuel-burning appliances, fireplaces, generators and cars. It can be deadly. Install carbon monoxide alarms on every level of the home and near sleeping areas, have fuel-burning appliances serviced regularly, and never run a generator, grill or car engine indoors or in an attached garage.

Volatile organic compounds

Where they come from: paints, varnishes, cleaning products, air fresheners, new furniture, carpets and some building materials. Formaldehyde, a common VOC, is released by some pressed-wood products.

What helps: choose low-VOC or zero-VOC paints and products, ventilate well during and after painting or renovation, store solvents and paints outside living areas, and let new furniture air out before bringing it in if possible. Fragranced air fresheners and plug-ins add VOCs rather than cleaning the air. Standard HEPA filters do not remove gases; filters with substantial activated carbon can help somewhat.

Radon

Where it comes from: a naturally occurring radioactive gas that seeps from the ground into buildings through cracks and gaps in foundations.

Why it matters: according to the EPA, radon is the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked.

What helps: radon can only be detected by testing. Inexpensive test kits are widely available, and the EPA recommends fixing homes with levels at or above 4 picocuries per liter. A radon mitigation system, usually a vent pipe and fan that draws gas from beneath the foundation, is installed by certified professionals.

Mold and humidity

Where it comes from: leaks, condensation and high humidity, especially in bathrooms, kitchens and basements.

What helps: the EPA recommends keeping indoor humidity between about 30 and 50 percent, ideally below 60 percent. An inexpensive hygrometer shows the level. Use bathroom and kitchen exhaust fans, fix leaks promptly, dry wet areas within 24 to 48 hours, and use a dehumidifier in damp basements. Very dry air in winter can be eased with a humidifier, which needs regular cleaning to avoid growing mold and bacteria itself.

Allergens

Dust mites, pet dander and pollen trigger allergies and asthma for many people. Washing bedding weekly in hot water, using allergen-proof mattress and pillow covers, vacuuming with a HEPA-filter vacuum, and keeping pets out of the bedroom reduce exposure. Keeping windows closed on high-pollen days and showering before bed in allergy season also help.

Cleaning and household products

Some everyday habits reduce indoor pollutants at the source. Vacuuming with a sealed vacuum that has a HEPA filter, and damp dusting rather than dry dusting, keeps fine dust from being stirred back into the air. Choosing fragrance-free or low-VOC cleaning products, paints and adhesives cuts chemical emissions. Store paints, solvents and fuels in a garage or shed rather than inside the living space, and keep containers tightly sealed. Air fresheners and scented candles mask odors rather than removing them, and many release VOCs or fine particles of their own.

Houseplants: a common myth

Houseplants are often promoted as natural air purifiers, largely based on a 1989 NASA study that tested plants in small sealed chambers. Later analysis, including a 2019 review by researchers at Drexel University, concluded that in a real home you would need an impractically large number of plants per square meter to match the effect of ordinary ventilation. Plants are pleasant to live with, but they are not a substitute for fresh air or filtration, and overwatered soil can encourage mold.

Choosing an air cleaner

Look for a portable air cleaner with a true HEPA filter and a clean air delivery rate, or CADR, suited to the room. A common guideline from the Association of Home Appliance Manufacturers is a smoke CADR of at least two-thirds of the room's floor area in square feet, so a 150-square-foot room needs a CADR of about 100. Run it continuously on a setting quiet enough that you will keep it on, and replace filters as directed. Avoid devices that intentionally produce ozone, which is a lung irritant, and be cautious about ionizers and other technologies that may produce ozone as a byproduct. A do-it-yourself Corsi-Rosenthal box, made from a box fan and several MERV 13 filters, became popular during the COVID-19 pandemic as a low-cost option, and testing has shown it can filter air effectively.

Ventilation basics

Opening windows, even briefly, dilutes indoor pollutants when outdoor air is clean. Run bathroom fans during and after showers, and kitchen hoods during and after cooking. Some households use inexpensive carbon dioxide monitors as a rough indicator of whether a room is well ventilated, since CO2 builds up when many people share a closed room. Newer, tightly sealed homes may benefit from mechanical ventilation systems that bring in fresh air while recovering heat.

When to get help

Persistent musty smells, visible mold over a large area, frequent headaches or respiratory symptoms that improve when you leave home, or high radon results are reasons to call professionals, such as certified mold remediators or radon mitigation contractors, and to talk to a doctor about symptoms.