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Story highlights

Beijing issues "orange" alert, highest this year

Residents in areas of northern China told to stay indoors

U.S. Embassy says AQI is "beyond index"

CNN  — 

Residents in northern China have been told stay indoors after air pollution in Beijing and neighboring regions rose to hazardous levels.

China’s state-run Xinhua news agency said that authorities issued an “orange” alert on Sunday, the highest of the year and second only to red.

More than 2,000 factories in polluting industries have been ordered to suspend work and schools have been ordered to halt all outdoor activities. At least one primary school in Beijing canceled all classes on Tuesday.

READ: Welcome to Baoding, China’s most polluted city

A sense of humor is helping Beijingers cope with the smog. Some took to social media to share photographs with their invisible landmarks penned in.

On Tuesday, the U.S. Embassy’s Air Quality Index (AQI), which measures hazardous PM2.5 pollutants, stood at 531. This was slightly lower than Monday’s 568 reading but still classed as “beyond index.”

A reading of more than 100 is unhealthy for sensitive groups.

The smog alert comes as global leaders meet in Paris for the COP21 climate summit. President Xi Jinping is expected to meet with President Barack Obama on Monday morning.

China, which is the world’s largest greenhouse gas emitter, aims to have its emissions peak by 2030.

Most of the country’s carbon emissions come from burning coal – to heat homes and to fuel power plants – and this spikes during the cold winter months.

The smog is expected to disperse on Wednesday when a cold front arrives.

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How climate change affects China
02:09 - Source: CNN

READ: China defends use of coal

CNN’s Shen Lu contributed to this report