Indoor Air Pollution: What Are the Major Air Pollutants at Our Home?

More and more people are paying attention to the air quality issue now, and the air monitoring system is becoming more and more perfect. We spend most of our time indoors, yet we don't know much about indoor air quality.

In the last few years, scientists have only just begun to pay attention and investigate the quality of indoor air, and the chemicals that contribute to indoor air pollution.


What Are the Major Indoor Air Pollutants?

Most indoor air pollutants come from the things we use and do indoors, which means that the composition of indoor air is often very different from that of outdoor air.

Indoor sources of pollution include gas stoves and wood-burning stoves, which emit a mixture of harmful gases, including nitrogen dioxide (NO2) and carbon monoxide (CO).

Candles are also a polluter, with one hour of burning producing enough NO2 to increase indoor concentrations close to the World Health Organization (WHO) limit of 200 micrograms per cubic meter (µg/m3).


These combustion sources also release tiny particulate matter (PM), and when concentrations are high enough, NO2 and PM can cause respiratory and cardiovascular diseases, while exposure to high concentrations of CO can be fatal.

Other sources of indoor pollution include household materials such as paint, vinyl flooring and soft furnishings, as well as personal care products, cleaners and air fresheners, among others.

Almost every item in your home releases potentially dangerous volatile organic compounds (VOCs) into the surrounding air, including the most familiar of all, formaldehyde, which is a respiratory irritant and carcinogen.


Studies on Indoor Air Chemistry

The first in-depth study of indoor air chemistry began in June 2018, when 65 scientists use a three-room laboratory in Texas to launch a project called HOMEChem.

Armed with $4.5 million worth of equipment, they spent a month cooking, cleaning and doing other routine tasks while using analytical instruments to measure the chemical emissions in the room.


They prepared a Thanksgiving dinner in the room, and the researchers found that when cooking roast Turkey and various toppings, NO2 concentrations peaked at about 200 µg/m3 due to emissions from gas stoves.

That's not all. They found that simple foods pose a greater threat, with stir-frying leading to a maximum concentration of PM10 (particulate matters smaller than 10 microns in diameter) of about 350 µg/m3, as a result of the combination of cooking oil and particles in food. The WHO limit for PM10 (24-hour mean) is 45 µg/m3.

Fried eggs, sausages and tomatoes (among other foods) produced the highest levels of PM2.5 (fine particulate matters less than 2.5 microns in diameter) of about 200 µg/m3, compared with the WHO limit of 15 µg/m3 (24-hour mean)

Know that PM2.5 may be more harmful to our health than PM10 because they are more likely to enter the respiratory system.


Chemical Reactions in the Indoor Air

Once these particles are released into the air, they don't stay in their original form for long because they react with other chemicals to form new ones.

Reaction rates are often different indoors and outdoors. For example, the light-driven reaction happens more slowly indoors because glass blocks a lot of UV rays and indoor lights tend to have lower UV levels.

Still, these reactions can happen. For example, there is enough light in the room for formaldehyde to react with oxygen and form two hydrogen peroxide (HO 2) radicals and a carbon monoxide (CO) molecule.

Hydroperoxyl radicals can form other oxidants that react with indoor volatile organic compounds, and there are hundreds of different reactions that form countless different compounds, some of which are harmful to health.

A more important driver of indoor reactions is the size of the available surface area. Surface area plays a more important role in indoor chemistry than outdoor chemistry because the surface area to volume ratio is much larger indoors than outdoors.

For example, fibers in wool surfaces (such as carpets and soft furniture) can greatly increase the acquisition surface area, far beyond the area occupied by such items in the room.

As a result, contaminants have more chance to settle to the surface and react with other chemicals deposited there.

In addition, tobacco smoke and e-cigarette vapors are a particular problem, with nicotine and other chemicals trapped in the air with exhaled gas and ending up on surfaces such as furniture or fabric.

This "secondhand smoke" can then react with indoor pollutants such as nitrous acid (HNO 2) to form new harmful substances such as carcinogenic nitrosamines, known as "thirdhand smoke".


Perhaps the most interesting surface scientists are exploring, however, is human skin. Ozone (O3) in the air reacts when it comes into contact with many of the oils and fatty acids on our skin, producing a range of secondary pollutants.

In our daily indoor lives, our skin surfaces not only eliminate ozone from the air, but also constantly react with other common substances.

For example, when bleach is mixed with water, it releases chlorine gas (Cl2), hypochlorous acid (HOCl), and other chlorine compounds into the surrounding air. And the oil on our skin contains unsaturated compounds with carbon-carbon double bonds, such as squalene (C30H50).

Hypochlorous acid reacts with the double bond to form chlorine compounds, which is why some people's skin can become allergic even without direct contact with bleach.



Thanks to these new scientific findings, our once-vague idea of indoor air pollution is starting to come into focus.

It is clear that even if we do not leave our homes, we are exposed to a mixture of chemicals and indoor air pollutants, especially in cities with so many houses that pollutants are most likely to build up indoors.

If you live in a city, it is essential to reduce your exposure to these pollutants. In addition to ventilation, an effective way is to use an air purifier at home, becasue a powerful air purifier can easily absorbs all the above air pollutants. 

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