A white mask looked dusty and had turned grey as if it had been left in a dustbin when in fact it had been worn for just one day by a resident in Lampang.
She posted the photo on her Facebook and it went viral on social media last week when the air quality index in the northern province reached 317 and the concentration of fine dust particulate matter (PM) 2.5 in the atmosphere was 207 microgrammes per cubic metre (µg/m³), four times higher than the safe threshold of 50µg/m³.
