How polycyclic aromatic hydrocarbons pollute the air
3 min read . Updated: 07 Feb 2020, 01:44 PM ISTPolycyclic aromatic hydrocarbons can associate with other particulate matter, like PM2.5, PM10, and make them more toxic
Recent findings from researchers at the Centre for Atmospheric Sciences at the Indian Institute of Technology (IIT), Delhi on the efficacy of a specific air purifier technology have revealed more about a class of pollutants known as polycyclic aromatic hydrocarbons, or PAHs.
Particulate matter in the ambient air consists mainly of PM 2.5 and PM 10 particles—tiny atmospheric particles that have a diameter of less than 2.5 and 10 micrometres, respectively. That’s about 3% of the diameter of a strand of human hair. PAHs are a class of chemicals and compounds that also contribute to air pollution. While PAHs occur naturally in items like crude, coal and gasoline, they can be produced by a variety of anthropogenic activities—like the burning of coal, oil, gas, wood, tobacco, garbage—including the incomplete combustion of organic matter. According to an open-access peer-reviewed chapter on IntechOpen, a publisher of open access science information, some natural processes, such as forest fires and volcanic eruptions, also contribute to PAHs in the ambient air.
When inhaled through particulates, PAHs can cause serious health risks—starting from irritation in the eyes and respiratory passages. Research studies have found links between prolonged exposure to high concentrations of PAHs and lung cancer in humans. Organizations like the US Environmental Protection Agency (EPA) and the International Agency for Research on Cancer have classified some PAHs as carcinogenic.
“We sampled the air during the peak pollution period in the 2018-19 winter season in three areas, one in Delhi and two in Ghaziabad, to get an idea about the entire National Capital Region (NCR). We did a full-scale analysis—what was there in PM 2.5, composition wise—and targeted three PAHs, which were dominant in the Delhi-NCR air during the specified time period," says Sagnik Dey, associate professor, Centre for Atmospheric Sciences. These were fluoranthene, chrysene and dibenz(a,h)anthracene, the latter two are classified as “probable carcinogens" by the EPA. A 2015 study by IIT, Kanpur had also found PAHs in Delhi’s air. According to a Press Trust of India report on the study, the presence of PAHs was alarming since it made particulate matter such as PM 2.5 and PM 10, present in huge amounts in the city’s air already, more deadly.
Dey was speaking on the sidelines of a recent event that discussed the effectiveness of an air purification technology—plasmacluster ion (PCI) technology—developed by Japanese company Sharp, which claims to remove PAHs from the air. This technology releases both positive and negative ions simultaneously into the air; these then recombine on the surface of pollutants floating in the air to create hydroxyl radicals, a type of oxidizing agent. These help in breaking down the pollutants and, thus, reducing their levels of toxicity.
Having identified the PAHs in the Delhi air, researchers at IIT, Delhi recreated the three compounds in lab conditions and observed them in two different set-ups for 10 days—one with the PCI technology, and the other without it. Concentrations of two of the three PAHs, fluoranthene and dibenz(a,h)anthracene, decreased significantly when exposed to the technology. There weren’t any huge reductions in the third compound.
PAHs can also latch on to, or bond with, other particulate matter, as the 2015 IIT, Kanpur study noted, making them a complex form of pollutants. Bipasha Ghosh, a postdoctorate fellow at IIT, Delhi, says while PAHs can also be found in the form of gases, like VOCs, their ability to associate with other particles makes them more harmful.
What are the chances of their presence in indoor air, given that PAHs are mostly found in ambient air through outdoor sources? Dey says that unless you are dealing with fully centralized air-conditioned houses or offices, most outdoor air finds it way indoors, a typical feature in Indian houses. “I can’t comment on the abundance of PAHs in the indoor air," he adds. “But most of the studies conducted in this area have shown that outdoor air infiltrates the indoor air. Then, we cannot completely deny the presence of PAHs in the indoor air."
