Farm fires in Punjab, local pollution and adverse weather conditions caused Delhi-NCR to breathe toxic air in the early months of winter, with PM 2.5 levels before Diwali 12-15 times above safe limit on a few days. However, those living in Delhi-NCR also breathed high levels of ultrafine PM 1 particles mid-October to mid-November, an interim analysis done as part of an ongoing NASA-funded study on pollution in north India and Indo-Gangetic plains revealed. Over the period of a month between mid-October to mid-November, PM 1 levels remained high in the range of 200-300 micrograms/cubic metre (ug/m3), said a team of scientists, from IIT Delhi, JNU and Delhi University. The analysis of PM 1 particles is important as no government agency monitors concentration of these ultrafine particles on a continuous basis, like they do for PM 2.5 and PM 10 pollution. There are no international standards or domestic standards that define the prescribed or safe limits of PM 1 concentration.