As the air quality in Delhi remained 'severe plus' on Tuesday, a report said PM2.5 pollutant levels was 60 times the World Health Organisation's recommended daily limit. WHO recommends that the annual average level of PM2.5 should not exceed 5 μg/m3, and the 24-hour average exposure should not be more than 15 μg/m3 for over 3-4 days per year.