Ozone — a pollutant that cause respiratory ailments, damage crops and forests — has been the highest in Delhi among the NCR cities of Gurugram, Faridabad, Noida and Ghaziabad for the last three years.Data submitted in the Lok Sabha on Friday by environment minister Prakash Javadekar revealed that “between 2016 and 2018, Delhi encountered at least 95 days, on which Ozone was found to be as one the ‘prominent pollutants’ in the city’s air”. “Usually, it is the level of particulate matter (PM10 and PM2.5) that remains high in Delhi,” an official of the Central Pollution Control Board (CPCB) said.Compared to Delhi, Noida experienced 49 days on which ozone was high, Gurugram recorded 48 days, Faridabad encountered 11 such days, and Ghaziabad encountered eight days of high ozone, in the last three years. Even though ozone in the upper reaches of the atmosphere is good for humans as it absorbs the sun’s harmful ultraviolet rays, when it is generated on the ground level, from vehicular and industrial fumes, ozone undergoes a chemical reaction in the presence of sunlight and turns harmful. “Rise in ozone levels is considered as one of the triggers for respiratory ailments. Irritation can occur in the respiratory system giving rise to coughs and an uncomfortable sensation in the chest. It may reduce lung function and make breathing difficult. There are no data available in the country to establish a direct correlation between mortality and high levels of ozone,” Javadekar said in the Lok Sabha. “This year, till May 31, Delhi has already encountered 23 days of high ozone. Faridabad has recorded the highest number of days this year so far, at 55. Gurugram and Ghaziabad recorded six and three such days, respectively, while Noida didn’t record any such day so far,” the CPCB official said. Safar, a pollution forecasting agency under the earth sciences ministry, had been issuing warnings of ozone pollution over the past two weeks. The Centre for Science and Environment, a Delhi-based NGO, released a report last week, which also showed that between April 1 and June 5, ozone levels were high on at least 16% days this year, compared to 5% days in 2018. The highest concentration in 2019 went up to 122 mpcm, which was 1.22 times higher than the eight-hour average standard. “This is a serious trend as ozone can have an adverse effect on those suffering from asthma and respiratory conditions. If this trend continues or worsens, the graded response action plan will also have to be enforced to address the precursor gases that form ozone — NOx, hydrocarbons etc — and crack down on vehicles and industry,” Anumita Roychowdhury, executive director (research and advocacy), CSE, said.