The heaviest smog of the year blanketed the Chinese capital Friday on the eve of the country's annual parliament.
The level of fine particulates hit the "hazardous" level of 452 micrograms per cubic metre overnight, according to the air quality index (AQI) by the US Embassy, and was at 405 micrograms at the time of writing.
The maximum safe level is 25 micrograms, according to the World Health Organization.
After relatively good air quality in January and February, the bad air returned just ahead the National People's Congress, which is expected to begin its yearly session on Saturday.
Beijing issued its first red smog alerts in December when the AQI hit 500, under a colour-coded system implemented in 2013.
The four-tier warning system has red as the most severe, followed by orange, yellow and blue. But last month, China made it harder to trigger a red alert.
The highest alert will only be issued when the daily average AQI is forecast to exceed 500 for a day, 300 for two days in a row or 200 for four days, according to the official Xinhua news agency.
The AQI on Thursday was over 300 on the index for the entire day.
Heavy smog is common in the winter months in northern China, where coal is widely used.