epa08109429 A man wears a face mask against air pollution in Bangkok, Thailand, 07 January 2020. Fine particulate matter (PM2.5), which can cause serious health issues, reached unhealthy levels in Thailand's capital, with the amount of fine particulate matter of 2.5 microns or less in diameter exceeding 100 in Bangkok and surrounding areas, according to the Pollution Control Department. World Health Organization's (WHO) guidelines put the safe concentration of pm2.5 at 25 micrograms per cubic meter in a 24-hour period, or 12 micrograms in a year.  EPA/DIEGO AZUBEL
A man wears a face mask against air pollution in Bangkok on January 7. EPA

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