Chinese authorities have raised the smog alert level to the second highest so far this year in the nation's capital Beijing.

The 'orange level' smog warning was declared on Sunday. This is the second highest possible level alert. It has reportedly prompted factories to close down and stop production.

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Health authorities said some pollution readings in some parts of the city recorded pollution level 17 times higher than the required safety level set by the World Health Organization (WHO).

The WHO has warned that if the smog readings continue to rise beyond the required level, those working in the capital and other major cities will soon get sick from pollution-related chronic diseases like asthma and lung cancer.

Chinese state media said the declaration of the orange alert level meant that factories and other industrial plants should reduce or completely close down their operations.

Sunday's orange level declaration likewise bans trucks from plying the roads of major Chinese cities. Authorities also ordered building sites to stop transporting materials and waste.

The US Embassy in Beijing reported Sunday noon that based on its air pollution monitor, the level of poisonous tiny particle PM 2.5 has reached more than 400 micrograms per cubic meter in the worst-hit cities, which is way too unsafe for humans and the environment.

The WHO considers 25 micrograms per cubic meter as a safe level.

Industries using coal and heating systems as well as dust gathering in construction sites contribute heavily to the thick smog enveloping parts of Beijing. The pollution condition is made more harmful with the humidity and a lack of wind.

Weather experts said Beijing will find relief on Wednesday with a cold front expected to arrive on that day. 

Tagssmog, 'orange level', Beijing Pollution, PM 2.5, US Embassy, 400 micrograms per cubic meter, coal-powered industries