The Chinese capital is notorious for its air pollution but even by those standards, this winter has been pretty bad.
Two weeks ago air pollution in Beijing hit record levels - four times what would be the normal unsafe levels.
After a brief respite, the 25 million residents of the Asian mega-city are again breathing in fetid air.
China's top leaders have acknowledged the problem, telling people it will take a long time to fix.
But those who have to breathe in a toxic cocktail simply going about their daily lives say they are not prepared to wait.
Residents are using a mobile phone app to compare the official Chinese government's reading with measurements from the United States embassy and decide whether it is safe to go outdoors.
Professor Pan Xiaochuan from Peking University Medical School leads a team that is measuring Beijing's air quality.
Government officials go to him for advice and lately he has been delivering the bad news.
"Such serious air pollution will of course have a major impact on the human body," he said.
"There's a rising death rate, an increase in cardiovascular diseases and respiratory system diseases.
"This smog is an alert to the government. It needs to put in a greater effort combating car exhaust and fired coal emissions."
Harmful particulates are drifting into Beijing from coal-fired power stations in Shanxi and inner Mongolia.
They get locked in by surrounding mountains and mix with the exhaust of 5 million cars.
Because air pollution travels and it is harder for poorer provinces to forgo economic growth, environmental groups want a regional strategy.