The fact that Delhi's air is laden with ultrafine particulates is undisputed. But does that automatically make it "toxic"?
A study has found that the ultrafine particulates in Delhi's air, mainly PM2.5 and PM10, are made of relatively less harmful chemical components.
Scientist Gufran Beig, project director of System of Air Quality and Weather Forecasting And Research (SAFAR) that undertook the study last December, told PTI that the chemical characterisation of particulates, meaning what they are made of chemically, primarily decides the toxicity of air.
According to a SAFAR analysis, in Delhi, around 7.6 per cent of the particulates are made of black carbon (BC).
Sulphate (SO4) particles stand at around 7 per cent, the larger presence of which would have heightened the toxicity of Delhi's air.
The agency, which comes under the Ministry of Earth Sciences, has found that the major share of the particulates, around 38 per cent, are actually made of aluminium and silicon oxides, present on the earth's crust, which are not as harmful as BC or SO4.
BC or SO4 act as aggravating factors when present beyond 15-20 per cent, Beig said. In fact, in Mumbai, where the volume of pollutants is less compared to the national capital, the percentage of BC is on the higher side, he said.
However, Beig cautioned against underestimating the gravity of the situation.
"The presence of toxic components may be less, but we have to keep in mind the sheer abundance of particulate matter in Delhi's air. So the volume of particulates, which spikes periodically, may be negating the potential benefits of having less harmful chemical components," he said.
He said even BC levels shoot up in phases, especially during the biomass-burning season, or even due to local factors such as burning of waste in the open. Black carbon or elementary carbon and SO4 are mainly products of incomplete or low temperature combustion.
According to SAFAR, following is the break up of particulates in Delhi's air:
Prolonged exposure to PM2.5 and PM10 beyond prescribed limits can cause harm to the respiratory system as the ultra fine particulates can find their way deep into the lungs and also enter the bloodstream.
The prescribed 24-hour-average of PM2.5 is 60 micrograms per cubic metre while the same is 100 in case of PM10.
"To assess the actual impact of pollution on an individual, factoring in the chemical characterisation of pollutants is crucial. One should not jump to conclusion purely based on figures of particulates captured by monitoring stations," he said.
Delhi had experienced a severe episode of smog last November when the Diwali fireworks had pushed pollution to an alarming level.
During that period, or in case of the episode of biomass burning, the air was likely to have been highly toxic due to a deadly cocktail of harmful respirable pollutants and gases, which bursting of firecrackers produce.