Smog in the Chinese city of Shanghai has hit its highest level since January, prompting schools to ban outdoor activities and authorities to limit outdoor work as polluted air spreads around the country.

Shanghai authorities have issued a yellow alert, advising elderly, young and sick residents to remain at home.

Last week, hazardous pollution levels in Beijing triggered the capital’s first red alert, meaning vehicles were ordered off the roads, classes were cancelled and heavy vehicles banned.

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On Tuesday, a curtain of grey smog fell over Shanghai, China’s business capital with a population of over 20 million.

It limited visibility and drove the city’s air quality index (AQI) above 300 — a level deemed hazardous on most scales and which can have a long-term impact on health.

The levels of PM 2.5 (dangerous tiny pollutants) hit 281, the highest since mid-January, according to data compiled by the US Department of State.

The US Government deems a level of more than 200 “very unhealthy”.

PM 2.5 particles are a major cause of asthma and respiratory diseases, experts say.

“Because of [the smog] my kid often gets sick, often has a stuffy nose and a cough,” said Valen Wang, 40, a full-time mother in Shanghai.

“At the moment, the pollution feels like it just keeps on getting worse, and all we can do is slow it down a little.”

– with agencies