Air quality index of Delhi is generally Moderate (101-200) level between January to September, and then it drastically deteriorates to Very Poor (301-400), Severe (401-500) or Hazardous (500+) levels in three months between October to December, due to various factors including stubble burning, fire crackers burning during Diwali and cold weather.[5][6][7] In November 2017, in an event known as the Great smog of Delhi, the air pollution spiked far beyond acceptable levels. Levels of PM2.5 and PM 10 particulate matter hit 999 micrograms per cubic meter, while the safe limits for those pollutants are 60 and 100 respectively.[8]