The ability of PM2.5 air pollutants to cause damage to cells increases sharply after concentration levels cross a certain threshold value, new research has found.
For Kolkata, where the study was carried out, this threshold value is about 70 micrograms per cubic metre (µg/m³), the study found. Once the PM2.5 concentration crosses this level, its toxicity — or the potential to cause damage to the human body — increases sharply, and continues to rise until the concentration reaches about 130 µg/m³. Toxicity stabilises after that, and further increases in concentration do not lead to an appreciable rise in damage potential.
The study, by Abhijit Chatterjee of the Bose Institute, Kolkata, and two of his former PhD students, Abhinandan Ghosh and Monami Dutta, is the first attempt at investigating how toxicity of air pollution changes with concentration levels in Indian cities. (‘Contrasting features of winter-time PM2.5 pollution and PM2.5-toxicity based on oxidative potential: A long-term (2016–2023) study over Kolkata megacity at eastern Indo-Gangetic Plain’, Science of the Total Environment, December 2024)
This is not to suggest that at concentrations below 70 µg/m³ in Kolkata, air pollution is benign and does not pose any threat to human health. Pollution is damaging at lower concentrations as well, but it becomes much more toxic after crossing the threshold value.
Why this happens
At lower concentrations, the body copes better with the adverse impacts of inhaling pollutants. After the threshold level, however, the body’s defence mechanisms are overwhelmed, and the pollutants are able to cause greater damage to cells, particularly the respiratory systems which are affected first.
When pollutants are inhaled, the body’s immune system tends to fight back through the release of Reactive Oxygen Species (ROS), which are chemicals used by immune cells to neutralise foreign substances. When larger concentrations of pollutants are inhaled, greater amounts of ROS are released.
The problem is ROS is damaging for the body’s cells as well. Therefore, as a natural counter-defence mechanism, the body produces another set of chemicals, called antioxidants, that protect the cells against ROS.
However, antioxidants are present in small quantities, and take time to build up. So, while they are able to effectively deal with smaller amounts of ROS, they are helpless when ROS is produced in large amounts.
This leads to an imbalance in the body, a situation called oxidative stress, which leaves the internal cells prone to damage from excess ROS. “The PM2.5 pollution level is well understood and established in every city in India… We wanted to see how the level of toxicity increases with the increase in pollution level,” Prof Chatterjee said.
“We know that when PM levels increase, there are several consequences, such as haze, dust, and visibility reduction. But how far does toxicity surge? We did not know. Though India has its standards of PM2.5 and PM10, we don’t have a threshold value in terms of oxidative stress in the human body. That’s where we wanted to determine the standard,” he said.
According to Prof Chatterjee, “When the PM2.5 concentration level exceeds about 70 µg/m³, its potential to create oxidative stress increases steeply, mainly because of the presence of some specific chemical components like those coming from biomass or solid waste burning. The components from vehicular emissions also help increase oxidative stress, but this is much less compared to biomass burning.”
The threshold levels at which toxicity sees a sharp rise is expected to vary from city to city, mainly due to the differences in the composition of the air pollutants. In some cities, vehicular emissions might be the biggest contributor to pollution; in other places, biomass burning might be significant.
Policy implications
Air quality standards are framed in terms of their concentration, not toxicity. In India, for example, a PM2.5 concentration level of 40 µg/m³, averaged over a year, is considered safe. On a daily basis, a concentration of 60 µg/m³ is considered safe.
However, the harmful impacts of air pollution on human health depend not just on concentration, but also on toxicity, which takes into account factors like chemical composition of pollutants.
In Kolkata, for example, the threat that annual PM2.5 concentrations of 50 or 60 µg/m³ poses is not going to be very different from the one posed by a concentration of 30-40 µg/m³. But alarm bells must go off once a level of 70 µg/m³ is reached. In other cities, this would be different.
This study can thus make the case for toxicity-based air quality standards, which can trigger targeted warnings/emergency actions once the threshold concentration is breached.
When pollutants are inhaled, the body’s immune system tends to fight back through the release of Reactive Oxygen Species (ROS), which are chemicals used by immune cells to neutralise foreign substances. When larger concentrations of pollutants are inhaled, greater amounts of ROS are released. (File)
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