Mongolians have their sent children into the countryside to escape choking winter smog as pollution exceeds the limits.
US Embassy data show annual average PM2.5 concentrations hit 100 micrograms in Ulaanbaatar, capital of Mongolia, in 2018.
They soared to 270 in December. PM2.5 in China's most polluted city of Shijiazhuang stood at an average of 70 micrograms last year, down 15.7 percent from 2017.
The World Health Organization recommends a concentration of no more than 10 micrograms.
About 60 per cent of Mongolia is covered by grassland, where the mining of copper, gold, coal and other minerals provides employment, while the Gobi desert envelops the South. But almost half the population live in Ulaanbataar
It is estimated that 80 per cent of Ulaanbaatar's smog was caused by coal burning in 'ger' districts, where thousands of rural migrants, used to a nomadic lifestyle, have pitched huts.
World Health Organisation(WHO) estimates air pollution causes more than 4,000 premature deaths a year.
A joint study by the UN International Children's Emergency Fund (UNICEF) and Mongolia's National Centre for Public Health said children living in one smog-prone district of Ulaanbaatar had 40 per cent less lung function than those living in the countryside.
In a crowded township more than 40 miles from Ulaanbaatar, Jantsandulam Bold's five grandchildren are breathing more easily after fleeing the capital.
'Fresh air and sun are most important for kids to grow healthy and robust,' says Jantsandulam, 57, making milk tea for her grandchildren in her home, a thickly padded felt hut known as a 'ger, or in Russian, a 'yurt'.
The temperature is expected to drop to minus 32 degrees Celsius (minus 26F) in Ulaanbaatar on Monday, as residents burn coal and trash to try to keep warm and concentrations of smog particles known as PM2.5 routinely exceed 500 mg per cubic metre, 50 times the level considered safe by the WHO.
'Air pollution aggravates respiratory diseases and children under five are most vulnerable as their organs are still not mature, said Bolormaa Bumbaa, a doctor at Bayangol District's Children's hospital in Ulaanbaatar.
At the Bayangol hospital, Ulzii-Orshikh Otgon, 34, was forced to bring her 10-month-old daughter Achmaa in with pneumonia for the second time in a month.
'I believe it's because of the pollution,' she said, adding that home air purifiers did little to help.
Families have already set up a pressure group known as Moms and Dads Against Smog, but the protests were ignored said Mandakhjargal Tumur, a group coordinator.
'I don't believe the government will do enough to reduce pollution in coming years,' she said. 'That's why we are now focusing on raising awareness.'
Mongolia, a former Soviet satellite landlocked between Russia and China, has invested public money and foreign aid to tackle pollution, but the improvement has been slow, with residents saying inaction has been compounded by a corruption scandal that has paralysed parliament.