As populations grow and cities expand, air quality becomes more of a concern among those who worry about respiratory health. High-density cities like New Delhi and Beijing have publicized the extreme effects of excessive air pollution. Frightening images of smog-laden urban jungles display pollution so bad that visibility is often reduced to less than a mile. Other photos of gridlocked traffic or industrial plants spewing smoke are anxiety inducing for anyone who sees them.

Less attention is given to the quality of the air people breathe indoors, but it is no less important. Unfortunately, it just isn’t as visible. It’s tempting to believe that we can escape polluted air in the comfort of our homes, but it isn’t a safe assumption. In some cases, indoor air quality can actually be worse. Let’s explore what exactly pollutes indoor and outdoor air.

What pollutes outdoor air?

The National Institute of Environmental Health Sciences tells us there are a variety of outdoor pollutants, both natural and human-made. These include:

• Particulate matter (PM) – These pollutants consist of chemicals like nitrates and sulfates. PM are produced primarily from smoke in processes like fossil fuel combustion, burning tobacco, and wildfires. PM becomes particularly dangerous when it is very fine, such as PM 2.5, which is 30 times thinner than human hair. It is extremely easy to inhale fine PM without realizing it, and it causes the most health effects of all pollutants.
• Noxious gases – Carbon dioxide, carbon monoxide, nitrogen oxides, and sulfur oxides are produced as a byproduct of industrial and motor vehicle processes and emissions.
• Ozone – Commonly known as smog when at ground levels, ozone is produced by cars as well as other industrial processes.

What pollutes indoor air?

Many outdoor pollutants cause problems indoors too, but the unique nature of indoor environments makes them work a little differently. Some common indoor pollutants include:

• Particulate Matter (PM) – While less commonly produced indoors, PM can be produced by burning things indoors, like in a fireplace or candle. Cooking can also produce inhalable dust. If you work in a garage, you are at higher risk of inhaling matter from wood or sandpaper.
• Cleaning and care products – People commonly store and use toxic cleaning chemicals like bleach indoors. Beauty products, paint, and other household chemicals can also pollute the air.
• Building materials – While used much less commonly now, older homes often contain toxic chemicals like asbestos, formaldehyde, and lead in their building materials. These are extremely dangerous and have been known to pose severe health risks. There are many more chemicals that can be released from sources all over the home.

The unique problems of indoor air quality

Indoor air isn’t just limited to what we breathe inside our homes. The truth is, most people breathe indoor air for most of their lives. Whether you are in your house, your workplace, your car, in public transport, or anywhere not fully outside, you are breathing indoor air. There are many ways this air quality can be reduced.

Unlike outdoor air, indoor air is subject to degradation from a lack of ventilation. Since the advent of climate control technology like heating and air conditioning, far less air is exchanged between outdoors and indoors. Without proper filtration and ventilation, outdoor pollutants can make their way indoors, where they will represent a far larger percentage of the total volume of air.

Modern homes, especially those in particularly hot or cold climates, use insulation technology that drastically reduces leakage in order to cut down on energy usage and costs. While this is in some ways advantageous, it can lead to disastrous air quality.

Some substance occur safely naturally outdoors but become dangerous when concentrated indoors. These substances, which can include animal dander, dust fibers, waste from household pests and pollen grains, can cause allergic reactions or asthma when inhaled in large quantities. Mold can also grow indoors, putting your lungs at risk of contamination that is far less likely when encountered outdoors.

The interconnected nature of indoor and outdoor air quality

Escaping polluted outdoor air is not as simple as simply going indoors, and the opposite is true too. The age of your home and where you live in the world both have an effect. The previously mentioned insulation technology used in modern American homes does a decent job preventing outdoor pollutants from getting inside, despite trapping indoor pollutants inside as well. However, the same cannot be said about older US homes. A Chinese study found that over 50% of fine particulate matter in homes came from outdoor sources, regardless of whether windows or doors remained consistently closed.

The finer a particle is, the more easily it can invade your home. Finer particles also are more easily able to enter your bloodstream, making them more dangerous in multiple ways.

Proximity to pollutant sources also has a major effect. If you live in a smog-ridden city, you are more likely to have poor indoor air quality. Even if your city isn’t particularly polluted, living close to highways or industrial districts poses a greater risk.

In some cases, indoor air quality can actually be worse than outdoor air quality. Even just in the US, the EPA estimates that indoor air quality can be up to five times worse.

What can we do to improve our air quality?

Improving outdoor air quality is highly reliant on systemic change, so it can be a daunting task for an individual. Writing to your local politicians and reducing your personal emissions can help on a small scale.

Improving indoor air quality, especially home air quality, is much more attainable. Ventilate your home as often as possible. If your region issues air quality alerts, ventilate at times when pollution is at a low. Keep your windows closed during periods of high pollution as well. You can also prevent the build-up of moisture or dust by cleaning up spills and wiping down surfaces as often as possible. General cleanliness also deters pests, who create more pollutants.

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