Is Air Pollution Worse Inside Your Home vs. Outside?

Whether you live in an urban or rural area, you may feel the effects of poor outdoor air quality. The same walls, doors and windows that protect your home from the elements could be an entry way for unwanted air pollutants like smog or pollen. Because so much time is spent indoors, the impact of indoor air pollution could be greater than what you would experience outside. This article will discuss the components of indoor air pollution, why concentrations could be worse indoors and the correlation between indoor and outdoor air quality.

What contributes to poor indoor air quality

The walls, doors, and windows of a house are essential for comfort, whether to let in the breeze or keep out the heat. They provide shelter from the rain and a barrier from outdoor habitats. The need for indoor spaces was felt thousands of years ago, and is just as necessary today: a roof over your head, a floor below and walls to the sides offer essential protection. Yet oddly enough,  the design of modern homes may have drawbacks for indoor air quality.

Buildup of indoor air pollutants

When thinking about your total exposure to air pollution, the pollutants you encounter in your home play a large role. Most people breathe in more indoor air than outdoor air; one researcher writes, “More that half of the body’s intake during a lifetime is air inhaled in the home” (Sundell, 2004).

Some of us may spend up to 90% of their time indoors, especially children and older adults. Therefore, the quality of indoor air  may be even more important than outdoor quality because of exposure time to pollution. According to the EPA, indoor air quality can be two to five times worse than outdoor air quality, especially with regards to airborne chemicals.

Poor ventilation

Though air tends to pass across environments, indoor spaces create environments that are different from the outdoors. Particularly over the past hundred years, indoor environments exchange less air with the outdoors, effectively sealing in the home to create personal climates. Because there is not enough ventilation, concentrations of indoor air pollutants can build up from inside sources as they are not released into the greater outdoor expanse.

The design of modern homes to reduce air leakage (with improved insulation) may also lead to lower air exchange between the indoors and outdoors. The California Air Resources Board found in a 2004 report that new single-family homes in California were built so airtight that they had lower outdoor air exchange rates than building codes required. Out of the 108 homes examined in the study, 32% did not use their windows. Because of these and other reasons, nearly all the homes had formaldehyde concentrations that exceeded health limits.

What are the sources of air pollution indoors vs. outdoors?

The actions of organized factories and whole industries affect outdoor air quality, so it is much easier to regulate than indoor air. Indoor air, however, is dependent on the unique actions of the person or people who occupy the indoor environment. In addition, small concentrations of pollutants indoors can lead to greater health concerns because they do not blow away with the next breeze. Mold spores blowing away at the base of a tree are not concentrated like the same spores building up inside a crawlspace.

Common indoor air pollutants and their sources

Many factors contribute to the composition of indoor air, from what artificial products are brought into the house, to cooking habits, to preferences for keeping doors and windows open.

Fine particles generally come indoors from the burning of fuels like candles for light or wood for heat. These particles can also be produced by cooking, from the dust that accumulates on surfaces in the home or even from common hobbies that might include the use of sandpaper.

Toxic gases can build up in the interior environment of a dwelling. While the raw products of industrial combustion are much rarer indoors, there is still a danger of carbon monoxide from heating. Naturally occurring radon gas can appear in a home by rising from the ground beneath. Though usually found at lower levels indoors than outdoors, ozone can infiltrate the home from outside or be produced by ozone-generating air purifiers, water treatment systems in washing machines and vegetable washers, and facial steamers that use UV light. 

Household products and chemicals that are used to keep your home clean, as well as personal care products, may release dangerous airborne chemicals. Beauty products, paints, lacquers, pesticides, and disinfectants are just a few of the purchases that bring toxic substances into the home.

Materials used to build furniture, electronics and houses themselves can contain dangerous substances that end up in the air and in the lungs. Asbestos, formaldehyde and lead are the most common in building materials, but there is also a host of chemicals that can be released from mattresses, carpets and new clothing.

Outdoor-indoor allergens are substances that occur naturally outdoors but become concentrated indoors and can cause or exacerbate allergies and asthma. Allergens can come from the leavings of pests like cockroaches, mice, and dust mites. They can also be released by organisms in the form of pollen or mold. Almost every home has some sort of allergen floating around. Allergens are invisible particles found on the surface of pollen grains, dust fibers, pet dander, and a wide variety of other places. They react with some people’s immune systems to induce an immune response even if the person has not been infected by a disease-causing organism. Therefore, poor indoor air quality is particularly dangerous to people who suffer from allergies and asthma.

Tobacco smoke can be just as dangerous inside as it is outside. One in nine people who die from a disease caused by tobacco are not even cigarette smokers. Tobacco smoke can harm anyone who enters a room where tobacco was burning within the past two hours, or even anyone who comes into contact with surfaces on which tobacco smoke has settled.

Common outdoor air pollutants and their sources

According to the National Institute of Environmental Health Services (NIEHS) and the EPA, there are several types of air pollution that come from outdoor sources. 

Fine particles are mostly produced by the burning of wood and fossil fuels for energy. You can usually spot these substances at the source as smoke from a factory or exhaust from a car’s tailpipe.

Toxic gases like sulfur dioxide, nitrogen oxides, ozone and carbon monoxide all form from the burning of fuel for energy. Chemical vapors can arise from industrial manufacturing. While ozone is naturally produced by lightning and is beneficial to the planet in the upper atmosphere, ground-level ozone harms living things. 

Tobacco smoke is a category all its own, as nicotine and the byproducts of burning tobacco are common among human populations and can cause unique health concerns in adults and children alike.

Can outdoor air pollution affect indoor air quality?

As discussed above, if there are intense sources of pollution inside your home, perhaps during remodeling or while new furniture is off-gassing, levels will be much higher indoors that outdoors (given that outdoor air quality levels are relatively stable).

How the presence of outside pollution affects indoor air quality is more complex. While there is a connection between outdoor air and indoor air, there are multiple factors to consider and no easy way to describe to what degree your home is influenced by outside conditions (Bo et al., 2017).  At a basic level, modern homes in the US usually do a decent job of protecting their inhabitants from low levels of outside pollution sources. In areas of China, however, where PM (particulate matter) levels are very high, indoor air pollution (PM2.5) concentrations are significantly and consistently correlated to outdoor concentrations. A review of the literature by Lin et al. (2018) found that between 54-63% of indoor PM2.5 came from outdoor sources, even when windows were closed.

Most studies agree that finer particles are better able to infiltrate a home, and may enter older houses faster than new ones. Finer particles tend to have the greatest health effects because they can enter the lungs and sometimes the bloodstream. Your proximity to outdoor sources of air pollution (e.g., if you live near a freeway or in a city with high levels of traffic pollution) may affect your indoor air quality. The same goes for an extreme air pollution event like wildfire smoke, where particles and toxic gases may enter and windows should be shut properly according to safety instructions.

The time of year (e.g., allergy season) may also affect the correlation between indoor and outdoor levels. Seasonal allergens like pollen are more likely to enter your home when outdoor levels are high during the spring months.

Ozone also readily enters homes, and it can be a particularly dangerous substance to have in the home. Not only is ozone a lung irritant, but in indoor spaces it can cause more particulate matter to rise into the air by reacting with substances in your home.

When is indoor air quality worse?

Though everyone breathes the same atmosphere, different concentrations of toxic substances can be present in sealed indoor spaces compared to breezy outdoor spaces. This is not only because indoors is a sealed environment; it also has to do with the activity in the home.

As for the scientific consensus, the EPA says that indoor air may be up to 5 times worse than outdoor air for reasons discussed in this article. Some of the pollutants that you encounter only briefly outside can become concentrated indoors and cause lasting health effects.

How to reduce indoor air pollution

It is a good idea to take steps to keep your indoor air clean to avoid the health problems associated with poor air quality.

An easy way to keep your indoor air healthy is to ventilate your home on a regular basis, if conditions allow. If you live in an area of the world where there are air quality alerts, you will want to restrict ventilation to the night or other times when you know there is not an excessive amount of pollution outside. During pollen season or extreme air quality events like nearby wildfire smoke, it is best to keep windows closed.

Another method is to control the sources of pollution in your home. If you have the space, allow new products to off-gas outside your walls or avoid entering a room containing an off-gassing item for a few days. Be sure that dust and moisture are not gathering in the corners and cracks of your home, because dust and moisture are food and water for mold, dust mites and other contaminants.

Our solution

For the most part, our homes protect us from the emissions of industrial factories, but there are similar pollutants indoors that can be more varied and dangerous than those just on the other side of the window.

Along with reducing pollution sources,  a complete long-term solution for poor indoor air quality is to invest in an air purifier. The Molekule PECO technology is a revolutionary new technology that has been third-party verified and used in a peer-reviewed study. Unlike traditional air filters that simply trap pollutants on filters, the Molekule air purifier destroys mold, allergens and airborne chemicals, providing truly clean air to your home.

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