Beijing was hit by its worst smog in almost a year yesterday. Exacerbated by heavy fog, the dense, choking smog that had blanketed the capital for the better part of the past fortnight plunged to a new low yesterday morning. Although the capital is often hit by smog in winter, the past two weeks have seen one of the longest stretches of smoggy days in Beijing in years. Pollution readings, including those for the smog-inducing fine particles known as PM2.5, from the local environmental watchdog and the US embassy were well in "severely polluted" or "hazardous" levels for most of Thursday night and yesterday morning. The embassy's hourly pollution data, published on Twitter, showed a reading of 488 micrograms per cubic metre of air at 11pm on Thursday and an air quality index of 492. The hazardous band of its air quality index readings begins at 300 and tops out at 500. The Beijing Municipal Environmental Monitoring Centre's air pollution index surged to well over 400 in the city centre. The thick shroud of fog enveloped the capital from early Thursday afternoon, reducing visibility in the city centre to less than 500 metres and leading to severe traffic jams yesterday. Air quality ratings for most of the municipal monitoring centre's 35 stations across the capital showed severe pollution levels yesterday. As of 11am yesterday, the embassy data showed a PM2.5 reading of 377 micrograms per cubic metre of air and an index reading of 419. By comparison, the PM2.5 reading in Hong Kong's Central and Western district, among the most polluted on the island, was 60.7 micrograms per cubic metre of air at 9am, according to the website of Hong Kong's Environmental Protection Department.