By Isabel Rowbotham, Co-Editor in Chief

Published May 28th, 2022

It is easy to forget that we are not out of reach of pollutants inside our homes. Notably, a chemical byproduct of combustion found in house dust poses potentially carcinogenic risks to our health. A recent study focused on how children may be exposed to this contaminant originating from pavements, driveways and parking lots, and how such pollutants make their way into our homes.

It is always exciting when research gets shared on social media. Published in 2021, a paper on indoor pollution found in house dust was being tweeted by various organisations regarding the alarming rate of Polycyclic Aromatic Hydrocarbons (PAHs), which are particles that originate from incomplete combustion, in American homes. While the tweets lacked factual nuance, they made me curious about the paper, due to the use of the hashtag #coaltarfreeUSA.

So what is the connection between PAHs pollution and the use of a chemical seal coat used on pavements and parking lots? First, we need to understand where PAHs come from. Some of these particles can come from natural sources, but others arise from certain human activities. Outdoor PAHs can originate from the burning of coal, oil, natural gas and wood; while indoor sources include smoking, cooking, heating and household cleaning products.

In the last 30 years, there has been a significant global reduction in the levels of PAHs in the air with the decrease in coal burning and indoor smoking. Despite these trends, the study showed that concentrations of PAHs in indoor air were approximately 2.6 times higher than that in outdoor air.

‘Indoor [PAHs] sources include smoking, cooking, heating and household cleaning products.’

This revelation shows that indoor pollution may be more significant than previously considered. One of the leading theories of why indoor air and dust pollution is still prevalent is the presence of gas heating, tobacco smoke and coal-tar-based parking lot sealers in homes. This article focuses on the latter.

What are coal-tar sealers and what is the connection with PAHs?

Coal-tar is a black gooey liquid used to seal pavements of driveways, parking lots and playgrounds in the U.S and Canada. Coal tar is a known carcinogen, with more than 50% of its weight being PAHs. A small study published in 2010 focused on coal-tar sealers as a source of PAHs inside homes.

But how can these parking lot-derived particles be found inside homes? The 2010 study suggests we carry it in our shoes into our homes and some may be introduced through the air. The study collected settled house dust from 23 apartments and 11 parking lots in residential areas in Texas, USA. The study suggested that households with adjacent coal-tar sealer parking lots had PAHs levels 25 times higher than those without parking lots.

These scientists also measured other potential sources, such as burned candles, fireplaces, grills, wearing shoes indoors, smoking, carpeting and heating; and concluded that 48% of PAHs particle was influenced by the presence of coal-tar near homes.

These particles are released into the air through wear and tear by car tires and sometimes can infiltrate nearby water sources. The same research group also found that pavement dust containing PAHs pollutants has been found to contaminate urban water streams.

‘…households with adjacent coal-tar sealer parking lots had PAH levels 25 times higher than those without parking lots.’

How does PAH cause cancer?

PAH-derived reactive metabolites are significant contributors to lung cancer. These metabolites pass cell membranes through enzymes and react with DNA to cause genetic mutations. PAH can enter the body through smoking (90% of cases), but other sources include air pollution and grilled or barbecued meats.

One of the most carcinogenic PAH is benzo[a]pyrene (BaP), which can be found in tobacco smoke, diesel, motor vehicle exhaust, industrial byproducts and forest fires.

Mutagenicity tests were carried out to assess the potential for carcinogenicity in samples containing PAHs. Results found a positive association between the potential for mutations in PAHs of indoor dust.

Although the aforementioned study was shown to find BaP in their samples, it was tested for mutagenicity but will likely be assessed in future studies to better understand the role of BaP on indoor pollution.

‘Results found a positive association between the potential for mutations in PAH of indoor dust.’

Health risks in children

Children may be more vulnerable to PAH exposure because their immune systems are still under development, as well as their higher respiratory rate compared to adults. Moreover, children one to six years of age are exposed through digestion and dermal absorption of household dust, perhaps due to crawling and playing on the floor.

Altogether the risk of cancer in children exposed to PAH dust could be significant, further research is needed to determine this with certainty.

Air pollution containing PAHs can also come into indoor spaces

Additionally, another source of PAHs in the air that can affect children’s health is traffic, mainly from diesel emissions. A study published in February 2022 reported DNA changes in school children living near low traffic and heavy traffic areas across various cities in Malaysia.

Most samples collected for the study showed the presence of BgP (benzo[g,h,i]perylen), another type of PAHs associated with vehicles. But can it also be found indoors?

The study found that children living near heavy traffic showed higher levels of DNA damage. The researchers determined that the source of most significant pollution was outdoor pollution, penetrating indoor spaces through windows and becoming inhaled. It is possible that through this manner, PAHs can pose a cancer risk in children.

The risk of children who lived near highways was statistically significantly higher than those who did not. Moreover, there was an increased risk among children exposed to second-hand smoking from parents and those who consumed grilled food.

Some good news is that eating fruits can have a protective effect on the damage caused by PAH exposure. Fruits and vegetables are rich in antioxidants, vitamins and polyphenols that are associated with fewer DNA strand breaks (damage) and less oxidative damage.

‘This type of pollution can also pose a cancer risk in children, through PAH inhalation and DNA damage.’

New research into the health effects of PAHs is helping close the knowledge gap regarding the effects of these pollutants on foetal and child development. There is growing evidence that PAHs may contribute to an increased risk of lung cancer. More recent research released in 2022 also shows associations with psychiatric disorders and prenatal exposure to PAHs.

Despite decreasing trends in air pollution in the last 30 years, the threat PAHs pose to the health of children in developing stages has been vastly under-researched. Moreover, studies show no difference in the amount of PAH pollution between North America, Asia and Europe. Through international research and collaboration, there is hope to better understand this unforeseen threat.

If you live in the USA and are concerned about PAH pollution from coal tar sealers, you can sign a petition to Congress here.

Featured Image: Dillon Kydd / Note Thanun | Unsplash

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