Is Indoor Air Quality Really Worse Than Outdoor Quality

These days many newspaper articles covering air pollution in our homes are being published.

Many of them contain interviews with experts to inform a general audience about the risks that poor indoor quality can have .

A recurring claim in many of these articles is the fact that indoor air quality is anywhere between 2-7 times worse than outdoor air quality.

However, a major issue with these articles is that they very often don’t mention the studies on which their claims are based.

This is important since there are various types of air pollutants in your house which can have varying concentrations when compared to outside levels.

Also, the term ‘worse’ can have a number of meanings in the context of air quality.

Worse can refer to exposure, which is related both to the concentration of the air pollutants as well as the time you spend in the polluted air, or it can refer to the concentration of pollutants only.

This article will give you a more holistic and comprehensive idea of the kinds of pollutants you encounter in your homes and their concentrations compared to outdoor air.

The concentration of Indoor Air Pollutants

Focusing on individual types of pollutants will give you a better idea on what ways indoor air quality is better or worse than outdoor air.

A number of different air pollutants contribute to the overall quality of the air in your home.

Some of these are found exclusively in houses and will, by definition, have a higher concentration than outside.

Others, however, enter from the outside air and have indoor sources as well. These can possibly concentrate in your house over time.

Here is the Summary of Various Pollutants and their comparative concentrations –

PollutantIndoorOutdoor
VOCsHigherLower
RadonHigherLower
Particulate MatterSituation DependentSituation Dependent
Gaseous pollutantsLowerHigher

All of these are discussed in subsequent sections –

VOCs

VOCs, or Volatile Organic Compounds, are another class of pollutants which are generally considered to be indoor pollutants rather than outdoor pollutants .

This is because they are emitted extensively by materials used inside houses .

These materials include those used for building, such as tiles, carpeting, power cables, and pressed wood.

They are also emitted by things such as paints, aerosols, air fresheners, and other sprays. Certain activities such as cooking food can also release them .

VOCs are a group of chemicals and comprise many different compounds, with the most common ones being formaldehyde, benzene, and naphthalene .

Therefore, the exposure for VOCs varies according to each chemical, with some being more highly concentrated outdoors .

Generally speaking, however, repeated studies have shown that overall VOC concentrations remain higher indoors than outdoors.

In fact, home VOCs can count for between 66% to 78% of the total exposure, with the remaining exposure happening either outdoors or in other indoor areas such as schools or offices.

At-home VOCs are very often caused by items such as mothballs and air fresheners . These VOCs include chloroform and dichlorobenzene .

Smoking also releases also accounts for a number of VOCs alongside items such as new furniture . Thus, the concentration of these VOCs is higher indoors.

Outdoor VOCs include gasoline-related VOCs and VOCs which are generated by industrial activities and dry-cleaning .

These have lower concentrations indoors than outdoors. However, ultimately, only a few VOCs have higher concentrations outdoor than indoors.

A notable exception is carbon tetrachloride – 81% of exposure to that VOC happens outdoors.

Overall, though, concentrations of VOCs can be up to 3-5 times higher indoors than outdoors.

This leads to unique health problems specific to indoor or home environments such as Sick Building Syndrome which can include a headache as well as flu-like symptoms.

Summary – VOCs concentration can be 3-5 times higher indoors than outdoors.

Particulate Matter

Particulate matter is generally regarded as one of the two main components of outdoor air pollution and hence its concentrations are generally quite high outside.

It is produced by the burning of fuel in industrial plants, power plants, as well as from vehicle exhausts .

Indoors, the primary source of particulate matter is cooking especially when burning fuel to cook, although fuels such as LPG and natural gas tend to produce much less particulate matter .

Sweeping can also cause dust particles to become suspended in the air and smoking tobacco releases high amounts of particulate matter .

Studies about the concentration of particulate matter in houses have shown varying results depending on how low the outside concentration of particulate matter is.

If the particulate matter concentration outside is low (lower than 100 µg/m3) then particulate matter concentrations indoors tend to be higher as indoor sources predominate .

Generally, however, even in the most polluted cities of the world, such as Beijing, particulate matter concentrations remain roughly equivalent indoors and outdoors, with the indoor concentration being only around 4% lower than the outdoor concentration .

However, this does not mean that particulate matter cannot accumulate in your homes.

In fact, without proper ventilation, studies have shown that indoor air can have more than twice the concentration of particulate matter than outside .

In working environments, the concentration of PM can vary according to the ventilation systems available at your workplace.

A study in Delhi, for instance, found that concentrations of PM10 were as much as 6-10 times higher in commercial buildings and PM2.5 had between 1.5-2 times as much concentration .

The buildings monitored in the study were office spaces, restaurants, hotels, and cinema halls .

Summary – Thus, particulate matter concentrations are highly dependent on outdoor concentrations and they follow similar trends. However, due to poor ventilation and various indoor activities, it is easy for indoor particulate matter levels to spike above outdoor levels, especially when outdoor levels are low.

Radon

Radon is a radioactive gas that is produced naturally in the ground from the decay of uranium and is, hence, present in almost all soil.

In general, it is not a significant pollutant outdoors as it dissipates. It is present only in trace amounts in the ground and in outdoor water bodies such as lakes.

Its concentration is measured in Becquerel per cubic meter, where one Becquerel refers to one atom decaying per second .

The average outdoor concentration of radon is 5 Bq/m3 .

The indoor concentration is generally many times higher than the outdoor concentration due to non-radon-proof construction techniques or poor ventilation.

In a study done in Bangalore, the average concentration was around 25 Bq/m3 .

However, it changes significantly throughout the day and can go up to 42 Bq/m3 in the morning and fall to 15 Bq/m3 in the afternoon .

At these levels, it is not a significant threat to your health and its risk to your health is calculated by the WHO as being 0.3%, which means that long-time exposure can cause lung cancer in 3 out of 1000 people .

Summary – Radon levels at home are higher than what they are outdoors.

Gaseous pollutants

Gaseous pollutants include a large number of compounds such as nitrogen oxides, sulphur oxides, and carbon oxides. Each of these has different effects on your health and have diverse sources.

Due to their varying sources, they have different mechanisms of entering and getting concentrated in indoor spaces. As such, the main components of gaseous pollution will be treated separately.

Nitrogen oxides

Nitrogen oxides are a group of 7 gases made up solely of nitrogen and oxygen .

However, the most common of these gases is nitrogen dioxide or NO2 and most studies focus on the concentration of this gas over the other nitrogen oxides.

The sources of nitrogen oxides are mainly from outdoor activities. Cars are major sources of NO2, which is emitted as exhaust fumes .

Diesel cars tend to emit much higher amounts of nitrogen oxides than cars which run on other fuels .

In indoor settings, the main sources of nitrogen oxides are cooking without proper ventilation and cigarette and tobacco smoking .

In areas where cars are the main sources of nitrogen oxides, such as in urban areas and large cities, the concentration of NO2 outdoors tend to be significantly higher than indoor concentration.

A number of studies have established this fact, although the exact ratio of the concentrations varies from city to city .

Indoor concentrations can be as low as 50% the outdoor value although they can go as high as 88% the outdoor value .

Furthermore, since cars are the main sources of nitrogen oxides, weekends tend to have lower levels of NO2 pollution overall because fewer cars are on the roads than on weekdays .

Summary – Outdoors dominate the concentration of Nitrogen ofides.

Sulphur oxides

Out of sulphur oxides, the most common one is sulphur dioxide or SO2. Other sulphur oxides are often produced through reactions of SO2.

However, not many studies have focused on the relative levels of sulphur oxides present indoors and outdoors.

Many studies which give this data generally focus primarily on particulate matter and sulphur oxide data is collected on the side .

That being said, the general trend that has been shown by few studies so far is that SO2 concentrations are lower indoors than the outdoor equivalent.

The ratios of the two vary widely in the studies but generally, SO2 levels are generally above 70% of outdoor levels.

Summary – General trend is that the indoor concentration of sulphur oxides are lower than outdoors

Oxide of carbon

Oxides of carbon include CO2, the well-known greenhouse gas, and CO, or carbon monoxide which is a highly poisonous gas.

CO2 concentrations in indoor areas, both in homes as well as other areas such as schools or workplaces, are generally extremely low .

Carbon monoxide, on the other hand, has both indoor and outdoor sources, unlike CO2 which has primarily outdoor sources caused by the burning of fuel and other materials.

Outdoor sources include cars exhausts. Because of this, areas with heavy traffic have higher concentrations of carbon monoxide .

In these areas, indoor levels can easily exceed outdoor levels as CO accumulates inside homes .

In general urban areas, however, CO levels tend to stay lower than outside levels .

In urban areas in India, at-home sources of carbon monoxide do not really count since they include things such as burning of wood indoors and clogged chimney.

Summary – Thus, gaseous pollutants generally tend to have higher concentrations outdoors than indoors, or the exact concentrations vary according to the season, the outdoor sources, the indoor sources, as well as the season.

Other air contaminants such as allergens and biological contaminants do not have many studies pertaining to them. Furthermore, they differ a lot qualitatively.

For instance, indoor allergens generally include animal dander and insect droppings while outdoor allergens are generally restricted to pollen.

As such, there are no real comparative studies for them.

Conclusion

Indoor air quality is affected by a number of factors and pollutants. These pollutants have different sources and hence they have different concentrations in the air.

Pollutants such as radon and VOCs are generally exclusive to indoor spaces since their concentrations outdoors are almost negligible, although there are a few exceptions among VOCs. As such, the exposure to them is highest indoors.

Particulate matter is also either higher indoors or at the same level as outdoor concentrations. For gaseous pollutants, the overall levels of nitrogen oxides, sulphur oxides, and carbon dioxide are generally lower in indoor spaces.

The exact ratios vary according to the pollutants.

Thus, air quality indoors does tend to be worse than outdoor levels, with maximum exposure to radon, VOCs, and particulate matter taking place indoors and a significant exposure to gaseous pollutants also happening in such areas.

This is consistent with many studies and researchers’ warnings of needing to consider air quality more holistically since outdoor air pollution is only one facet of it.

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