Smog has returned to hazardous levels in China's capital, even as the government enforced unprecedented measures to cope with the severe air pollution.
As the authorities thinned at least two million private cars from Beijing's roads, stopped construction work and shut down polluting industries, levels of harmful PM2.5 particles continued to rise throughout the day.
They exceeded 350 micrograms per cubic metre in parts of the city, according to real-time data compiled by the United States Embassy in Beijing.
That is more than 14 times above the safe limit set by the World Health Organisation (WHO), once again raising questions about China's ability to fight pollution.
The health hazard emerged again as officials were forced to defend issuing a pollution "red alert" for the first time on Monday - it had not done so under more severe conditions last week.
With all cars bearing odd-numbered plates not allowed out yesterday, the measures strained the city's public transport system, despite the authorities adding 800 buses and 50 subway trains to the network.
Local media reports said trains were packed, with the morning crush starting earlier than usual, as an estimated two million more people joined the commute.
Many schoolchildren also stayed home, with all schools urged to close.
Separately, the Ministry of Environmental Protection increased the number of inspection teams to 12, sending them to regions known for heavy pollution around Beijing.
Their task was to monitor polluters and ensure that emergency response procedures were being followed.
"Environment authorities must closely follow the situation, improve monitoring and forecasting, and guide local governments' emergency response plans," Environment Minister Chen Jining told reporters.
The decision to issue a red alert was praised by WHO's representative in China, Dr Bernhard Schwartlander, who said in a statement that it showed that Beijing is "taking air quality, and related health issues, very seriously".
But the red alert was questioned by many Chinese online, who asked if the government acted under political pressure after being heavily criticised last week for not issuing one.
The authorities denied this, saying that alerts are issued based on duration and not severity.
"A red alert will be issued only if heavy pollution lasts for at least three days," Beijing's environmental monitoring centre chief Zhang Dawei told state agency Xinhua. "But last time, the heavy pollution did not meet the requirements to issue a red alert."
Mr Zhang added that alerts are based on smog forecasts which "inevitably have discrepancies with real life" and that the authorities will continue to refine the system.
The assurances give scant comfort to frustrated residents such as aftersales manager Wu Xuelian, 38, who has asked to take leave from work today.
"I need to take care of my kid at home because she's not in school. Anyway it's inconvenient for me to go to work because I'm not allowed to drive my car," she told The Straits Times.