The international climate deal reached in Paris earlier this month is "the best chance we have to save the one planet we have", U.S. President Barack Obama said. China, the world's biggest polluter, also hailed the deal, as did India. The fact that the air pollution in world cities, particularly the biggest polluter China, reached alarming levels, proves the urgency of the situation in which humanity suffers. Beijing residents woke up to a white Christmas Friday morning but with the sky obscured by thick toxic smog rather than snow after more than 100 million people across China had been warned to stay indoors.The capital and surrounding parts of northern China are regularly blanketed in deadly pollution associated with heavy industry and an increase in coal consumption during the winter months. Counts of PM2.5 - harmful microscopic particles that penetrate deep into the lungs - in Beijing peaked at 620 micrograms per cubic meter as of early Friday, according to data from the US embassy.The World Health Organization's recommended maximum exposure is 25 micrograms over a 24-hour period."If this only happened a few days a year, I'd put up with it, but it's paralyzing for it to be like this every day!" said one angry social media user on China's Twitter-like Weibo platform. "Is this the new normal?"The city posted contradictory alerts for Christmas day, with the Beijing Meteorological Service issuing an orange alert requiring factories to limit expelled pollutants and schools to cease outdoor activities. As for the employees, it's not only about salaries, promotions and career prospects - many companies in China have found they need to offer cleaner air within their offices to lure and retain staff. Air quality has deteriorated badly in China's north and east, including in Beijing and Shanghai. Companies, especially multinationals, based in these cities are spending tens of thousands of dollars to install air filtration systems and real-time pollution monitoring devices in their offices."If a company is willing to reduce pollution inside, it shows it is responsible and will have good growth," said Shanghai resident Yao Hui, who has decided to leave a Chinesefurniture company where she has worked for four months after finding its office had the highest category of pollution on a measurement device she used.Staff turnover rates are relatively high for multinationals based in China, and air pollution has been cited as the top challenge in luring and retaining talent among such companies,according to the latest business confidence survey by the European Chamber of Commerce. Beijing has already issued two "red alerts" for smog this winter, which see cars taken off roads and schools and factories closed.And Indian cities don't get as much attention as Chinese cities do with respect to air pollution, said Anmol Vanamali, a senior fellow at the Worldwatch Institute. Yet, it is clear that Indian cities are just as, if not more, polluted. According to the World Health Organization's data from 2014, Delhi was ranked as having the highest PM 2.5 levels in the world. Smaller than 2.5 microns in width, PM 2.5 particles are particularly dangerous to human health because they are small enough that they can lodge into a person's lungs. This specific type of pollution has been linked to premature death in people with heart or lung disease and to heart attacksEven in Europe air pollution nightmare is at the door. Sarajevo authorities had to close down schools because of high pollution mixed with fog, and volunteers have begun distributing face masks to those who still decided to get out of the house despite warnings to reduce movement.Authorities said Wednesday it has been the longest siege of foul weather in decades.Flights have been canceled for the past few days because of fog, a common occurrence at Sarajevo's airport in winter.Rome, Italy's capital, also gripped by dangerously high pollution levels, reversed a decision Thursday to ban car traffic after the Christmas holidays hours after its rulers announced the decision. Prefect Francesco Paolo Tronca, a caretaker administrator for Rome, said there would be a restriction rather than a blanket ban Monday and Tuesday to better balance people's right to movement with environmental protection needs.In a scheme familiar to motorists in Rome and other Italian cities, traffic was to be limited according to the last digit of car number plates: odd numbers were to be allowed on the road one day, and even ones on the next.Facing similar smog problems, Milan Mayor Giuliano Pisapia took more drastic action on Wednesday, ordering a traffic ban in effect next week from Monday through Wednesday, accompanied by a 30-kilometre-per-hour speed limit for low emission vehicles exempted by restrictions.Rome and Milan are Italy's two biggest cities, with populations of 2.9 million and 1.3 million respectively. During the traffic restrictions, people in both metropolises will be offered reduced-price public transport tickets.The concentration of PM 10 and PM 2.5 particulates - tiny carcinogenic dust particles also associated with heart and lung diseases - has been dangerously high in Rome, Milan and several other Italian cities, including Naples and Turin. This has been partly due to the absence of wind and rain, which help sweep away pollution.Last month the European Environment Agency said PM 2.5 pollution was responsible for 59,500 premature deaths in Italy in 2012, an EU record the country shared with Germany. In the entire 28-member bloc the total early death tally was 403,000.A staggering US$ 1.6 trillion is the economic cost of the approximate 600 000 premature deaths and of the diseases caused by air pollution in the WHO European Region in 2010, according to the first-ever study of these costs conducted for the Region. The amount is nearly equivalent to one tenth of the gross domestic product (GDP) of the entire European Union in 2013.