Indoor air pollution poses significant risks due to prolonged exposure and limited air mixing. Combustion, including cooking on open flames, emits harmful particulate matter. Other sources include smoking, parking structures, and poorly ventilated areas. Chemical pollutants, such as solvents, VOCs, and formaldehyde, are found in various products. Vaping also releases volatile organic compounds. Indoor spaces harbor particulates like bacteria, pollen, dust mites, and allergens. Understanding these indoor air pollutants is crucial for safeguarding human health and working towards improved indoor air quality.

Indoor air pollution is an acute issue for a number of reasons. Firstly, we spend most of our time indoors, and so if the air quality is poor, it has disproportionate impacts. We also know that it is the very young, the very old and the chronically ill who spend proportionally more of their time indoors - the people who are most vulnerable to the environmental impacts. 

Secondly, indoor air pollution is so acute because, unlike outdoor space, homes are designed to restrict air mixing. We insulate, block out drafts, and, in the west at least, try as much as we can to hermetically seal our homes from either external heat or cold - this restricts mixing and builds chemical concentrations that we do not see outdoors. 

So what kind of indoor environments are bad for air quality?

Any indoor space that has combustion taking place - fires and stoves. Cooking on open flames remains common in much of the horn of Africa and Central and West Africa, in India and parts of China and Southeast Asia. Fuels such as wood, dung or charcoal when used indoors for cooking give out disproportionately high particulate emissions that are directly linked to respiratory diseases. 

But other indoor air quality, in all parts of the world, can hold harmful levels of air pollution, including parking structures polluted through the emissions of combustion, industrial areas, poorly ventilated kitchen areas, and anywhere that has a high level of smoking cigarettes or tobacco of any sort - the products of incomplete combustion, both chemical and particulate, will be high. 

Now let us consider the nature of the pollutants. 

We know anything involving combustion - high temperatures and open flames - will have nitrogen oxides, carbon dioxide, sulfur dioxides, as well as a great deal of particulate matter. But there are also many chemical sources we might not expect. In simple terms, if you can smell it, it probably indicates a gas phase compound, most of which at indoor concentrations can become harmful.

For example, the use of UV light allied with elevated temperatures and VOCs, such as found around photocopiers, UV air purifiers or sunbeds, can all generate high levels of ozone, a compound that oxidizes biological membranes such as the lining of our lungs. 

We find a great range of solvents, such as those found in paints and thinners, and cleaning products, in both domestic and commercial settings. These can be a source of many different types of volatile organic compounds. 

We find the associated pollutants of vaping are increasingly common- again, many volatile organic compounds in addition to the particulates generated in the vaping process. 

Carcinogens, such as formaldehyde, can be found in newly generated products like pressed wood materials, flat-pack furniture, and urea formaldehyde foam used in insulation, and also in older products, such a glues and binders, that are starting to decay. 

As well as the lead and asbestos we find used in so much building of the past century, we also find modern chemicals have proliferated that give out volatile organic compounds readily, such as air fresheners, scented candles, aerosol sprays, dry-cleaned clothing, any form of pesticide, weedkiller or insect repellent. 

In addition to the chemical, we also see in poorly ventilated indoor spaces a wide variety of particulates, from biological matter, such as bacteria and pollen, dust mites, pet dander and allergens, and aerosols of many varieties. Sizes vary, from 0.01 to 10 microns, all breathable and so all capable of having direct implications for human health.