The most recent online outpouring seems to have been set off on Sunday night, when the embassy published a PM2.5 air quality index reading above 500 – a level expats refer to as "Crazy Bad" – that contrasted sharply with the Beijing Municipal Bureau of Environmental Protection's description of air pollution over the weekend as "light."

While many Internet users have marveled at the discrepancy, others have been busy trading advice on the best types of facemasks to wear and what brands of air purifiers offered the best cost-benefit...