On Sunday, thousands of people who participated in the half marathon had to run the 21-km stretch through a thick cover of smog. According to the Central Pollution Control Board’s (CPCB) Air Quality Index (AQI), most monitoring stations showed particulate matter (PM) levels hovering between poor and very poor levels during the weekend. At R K Puram, whose AQI was at 373 Sunday morning, air quality was listed in the “very poor” category. PM 2.5 was the most prominent pollutant. At Anand Vihar, with an AQI of 395, PM 10 was the prominent pollutant. At Punjabi Bagh, also with an AQI of 395, PM 2.5 was the prominent pollutant. Mandir Marg with an AQI of 376, had PM 2.5 as the most common pollutant. While particulates dispersed during the afternoon, readings taken in the evening again showed particulates in the very poor range (between 301-400). According to the AQI, “prolonged exposure” to this air can cause respiratory illness. Meanwhile, at 8 pm, the US embassy’s independent monitoring index recorded an AQI of 295. This is the “very unhealthy” category, which can lead to, “significant aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; significant increase in respiratory effects in general population”. The embassy’s AQI is based on a PM 2.5 value of 244 micrograms per cubic metre. According to a cautionary statement issued by the embassy for air quality in this range, “People with heart or lung disease, older adults, and children should avoid all physical activity outdoors while everyone else should avoid prolonged or heavy exertion”. The meteorological department at IGI airport said Friday and Saturday had witnessed the worst smog spell so far this year, with visibility reduced to around 200 metres. The department’s director, Dr R K Jenamani, said, “Our analysis shows the smog is due to pollutants accumulated locally and has nothing to do with burning of crop residue in neighbouring states.” He added that the presence of moisture from easterly winds and relatively low day and night temperatures compared to October had provided weather conditions conducive to smog. While experts say pollutants will disperse as the winds grow stronger, melting of snow in the Himalayan range is likely to add to the fog — again resulting in more accumulation of pollutants.