Hu Li's heart sank when she realised that she could gauge how close she was to home by the colour of the air.

Driving 140km from Tianjin City to Beijing last week, she held her breath as the chalky-white horizon became a charcoal grey haze.

The 39-year-old businesswoman has lived in Beijing for a decade, and this past month, she said, brought the worst air pollution she had ever seen. It gave her husband a hacking cough and left her 7-year-old daughter housebound.

"I'm working here and my husband's working here, so we have no choice. But if we had a choice, we'd like to escape from Beijing."

A prolonged bout of heavy pollution over the past month, which returned with a vengeance for a day last week - called the "airpocalypse" or "airmageddon" by internet users - has fundamentally changed the way that Chinese people think about their country's toxic air.

The event was worthy of its namesake. On one day, pollution levels were 30 times higher than is deemed safe by the World Health Organisation. Flights were cancelled. Roads were closed. One hospital in east Beijing reported treating more than 900 children for respiratory problems. Bloomberg found that for most of January, Beijing's air was worse than that of an airport smoking lounge.

The smog's most threatening aspect is its high concentration of PM 2.5 - particulate matter that is small enough to lodge deep into the lungs and enter the bloodstream, causing respiratory infections, asthma, lung cancer, cerebrovascular disease, and possibly damaging children's development.

The WHO has estimated that outdoor air pollution accounts for two million deaths a year, 65 per cent of them in Asia. Yet the smog has become more than a health hazard in China - it has become a symbol of widespread dissatisfaction with the Government's growth-first development strategy.

Feelings of resigned helplessness have given way to fear, anger, and society-wide pressure to change the status quo.

The Lunar New Year usually coincides with clear blue skies - an estimated nine million cars depart from the capital, and its emissions-spewing factories shut down as workers go on holiday.

Yet the smog came back with a vengeance last Thursday.

Environmental authorities sent text messages to Beijing residents urging them to mitigate the pollution by refraining from the long-held holiday tradition of lighting fireworks.

According to state media, they took heed. Fireworks sales fell 37 per cent compared with last year.

"PM 2.5 and data measurement issues with regard to air quality have entered into mainstream Chinese life," said Angel Hsu, a doctoral candidate at Yale University.

Hsu has tracked use of the term "PM 2.5" on Sina Weibo, China's most popular microblog, over the past two years. In January 2011, it was mentioned about 200 times. Last month, the number soared above three million.

In China, PM 2.5 has acquired a symbolic weight to parallel its medical gravitas. Young internet users post photos of themselves wearing air filtration facemasks. Last year, Shanghai hosted a PM 2.5-themed rock music festival. A music video called Beijing, Beijing (Big Fog Version) went viral on video-sharing websites.

Experts say that the last month's pollution was probably caused initially by a cold snap, forcing huge use of coal, followed by a rare temperature inversion, which trapped emissions under a blanket of warm air.

Others say that it could be related to a prolonged period of high humidity, trapping particulate matter in the air.

Pollution levels depend heavily on the force and direction of the wind. A strong northeastern gust can blow the smog out to sea; a few stagnant hours are enough to make noon look like early evening.

The standard international measurement for air quality - the US Air Quality Index, or AQI - rates air quality on a scale of zero to 500. With experience, it becomes possible to guess the AQI in Beijing without looking at official readings.

One hundred correlates to a thin grey gauze hovering above the horizon. When the index hits 200, the sky is visible only in a small patch directly overhead. An AQI reading of 300 blots out the sun, smothering the city in drab uniformity. When the AQI reached 755 on January 12, the worst day on record, the air felt like industrial smoke - chemical-tasting and eye-watering.

On particularly smoggy days, the toxic cloud is visible in satellite photos. The worst of the last month's pollution stretched 1770km south, closing highways near the southwestern city of Guiyang.

When the smog clears, it does not simply vanish, but instead drifts to surrounding countries. January's smog spurred Japanese authorities to release health warnings to people living in the country's western cities. Traces of China's smog have been detected as far afield as California.

The Beijing municipal government has taken steps to curb the pollution, temporarily shutting down factories and ordering government cars off the roads.

While propaganda authorities used to quash reports of air pollution for fear that they could spark social unrest, Chinese newspapers were allowed to report freely on the crisis.

"I'm pretty optimistic this happened at the right time to prompt the most action possible," said Deborah Seligsohn, an expert on China's environment at University of California, San Diego.

President Xi Jinping took the reins of the Communist Party in November; incoming Prime Minister Li Keqiang has promised to make environmental protection a focus of his tenure. Beijing authorities hope to wean the city off coal and implement stricter vehicle emissions standards by 2016.

People have begun to take protection into their own hands. "People are starting to treat air purifiers as a necessary appliance," said Bi Xiuyan, a salesman for Amway. Observer

By Jonathan Kaiman