“In February, due to weather factors such as intermittent rains and a rise in temperature, particulate matter levels have been better, and there has not been any ‘severe’ day in February. But ozone levels have started rising very early in the year, probably due to the high temperature,” said a senior CPCB scientist. Experts said ozone levels go up with a rise in temperature. “Ozone is formed due to a photochemical reaction between other gaseous pollutants such as nitrogen dioxide with hydrocarbons,” said AnumitaRoychowdhury executive director, Centre for Science and Environment. Ozone has severe health effects. Doctors said it reduces lung function by inflaming and scarring the lining of the lungs. It can also cause chest pain and exacerbate existing respiratory diseases. “Ozone is the only pollutant which has a one-hour notified average safe limit of 180 micrograms per cubic metre. Along with carbon monoxide, it also has an eight-hour average safe limit of 100 micrograms per cubic metre. Inhaling ozone for one hour can be harmful for people with existing respiratory disorders,” said Dr T K Joshi, director of Centre for Occupational and Environmental Health at Maulana Azad Medical College.