Beijing was once again blanketed in smog yesterday, with the capital's authorities warning that the air pollution will last until at least Thursday and that people should take precautions when venturing outside. The environmental authorities issued a "yellow" smog alert at 3pm, advising adults to put on facemasks and recommending that children and the elderly reduce the amount of time they spend outdoors. The United States embassy's air quality index passed over 300 at about noon, a level deemed "hazardous" to health. Readings at monitoring stations in the city centre were between 250 and 300 in the afternoon, meaning the air was officially classified as "heavily polluted". Forecasters said the air pollution would probably get worse on Wednesday and Thursday as no wind or rain was forecast to disperse the smog. Under a yellow alert, according to a tiered warning system introduced last year, the traffic authorities increase water spraying to clean roads, while construction sites are asked to cover earthworks to reduce dust in the atmosphere. The public are also encouraged to use more public transport, while factories are asked to reduce polluting emissions. Watch: A view of Beijing's smog from atop the Forbidden City "These 'recommendations' are obviously not sufficient to limit emissions. Why doesn't the government issue a higher grade of alert if the smog is forecast to be that bad?" a Beijing resident asked on social media. The authorities in the capital have come under intense criticism from the public in recent weeks and accused of failing to take decisive action to tackle smog. In a week-long period of smog last month, concentrations of PM2.5, the small particles of pollutants deemed most hazardous to health, exceeded 500. The World Health Organisation recommends day-long exposure levels of 25, but the city government did not activate the highest red smog alert, which activates stronger measures to tackle pollution. The capital has enjoyed a few rare beautiful spring days over the past few days. One person able to enjoy a glimpse of sunshine in the capital was the US first lady Michelle Obama, who was pictured on Friday under blue skies at the former imperial palace the Forbidden City in the centre of Beijing.