Chinese struggle through 'airpocalypse' smog 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 140 kilometres 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 has ever seen. It gave her husband a hacking cough and left her seven-year-old daughter housebound. "I'm working here and my husband's working here, so we have no choice," she said. But if we had a choice, we'd like to escape from Beijing. A prolonged bout of heavy pollution over the last 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 levels deemed safe by the World Health Organisation (WHO). Flights were cancelled. Roads were closed. One hospital in east Beijing reported treating more than 900 children for respiratory issues. 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 per year, 65% 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, which came last Sunday, usually coincides with clear blue skies - an estimated 9m 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 on Wednesday. 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% 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 usage of the term "PM 2.5" on Sina Weibo, China's most popular microblog, over the last 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 face masks. One popular mask is hot pink; another looks like a panda bear. Last spring, 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. Who is searching in the fog? Who is weeping in the fog? Who is living in the fog? Who is dying in the fog," A man croons over images of cars crawling along smog-choked highways. 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 north-eastern 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 12 January, the worst day on record, the air felt like industrial smoke - chemical-tasting, eye-watering. On particularly smoggy days, the toxic cloud is visible in satellite photos. The worst of the last month's pollution stretched 1,100 miles south, closing highways near the south-western city Guiyang. When the smog clears, it doesn't 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. Shanghai's Environmental Protection Bureau has designed a cartoon accompaniment to its AQI readings - a pigtailed girl with big anime-style eyes, green-haired and smiling when the index reads "excellent" but maroon-haired and weepy when smog rolls in. "I'm pretty optimistic that 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. Seligsohn added that changes would take a while. "If Beijing were surrounded by cities that were doing the same thing that Beijing was doing, it would be fine, but it isn't," she said. A short drive from central Beijing, the landscape fans out into sprawling, dusty plains, where farmers burn coal to heat their concrete homes. Small factories there often escape the notice of environmental watchdogs. PM 2.5, she explained, is produced by four airborne pollutants - sulphur dioxide, nitrous oxides, volatile organic compounds, and black carbon - each of which would require its own slew of regulations to curb. People have begun to take protection into their own hands. "People are starting to treat air purifiers as a necessary appliance like a washing machine or computer," said Bi Xiuyan, a 56-year-old product salesperson for Amway. Bi has sold about 50 air purifiers in the last month, each of which costs £960, about twice the average monthly income for Beijing residents. "Everybody needs to breathe," she said. Louie Cheng, the president of Shanghai-based Pure Living China, a small company that tests indoor air pollution, said that the current situation boosted the company's web traffic 30-fold. "Literally you can see it - this isn't compared with a year ago, this is compared with a month ago," he said. Cheng said that he helped start the company three years ago when an expat friend with an asthmatic daughter couldn't find a local company to competently test his house for pollutants. His client-base has tripled since January, and now includes more than half of Shanghai and Beijing's international schools. "It's just hard to keep up with the demand," he said. Awareness of the problem has spread beyond major urban centres. Ma Shiying, who sells moist towelettes in the small coastal city of Weifang, Shandong province, heeded the government's warning and lit fewer fireworks this year. "Over the past few months, the whole world has begun to pay close attention to this problem," he said. It's become impossible for anyone to ignore. Yet interpretations of the issue vary. Eva Zhong, the head of exports for a fireworks manufacturer in Hunan province, said that the government's fireworks warnings were misplaced. "Fireworks are very innocent," she said. Car exhaust is a far greater problem. Despite the government figures, she added, her company's sales this year have been unscathed.