Shanghai's adjusted four-tier air pollution alert system, which lowered the criteria for alarm activation, was formally introduced by the environment authority yesterday.

Under the new criteria, the lowest blue alarm will be activated when both slight pollution (Air Quality Index reading between 101 and 150) and short-term heavy pollution (AQI over 200) are forecast for the coming 24 hours. Previously, a blue alarm was issued when moderate pollution (AQI between 151 and 200), and short-term heavy pollution were predicted for the following 24 hours.

The second-level yellow alarm will be triggered when the AQI is forecast for between 201 and 300 during the coming 24 hours, down from the previous 48 hours.