Fossil fuels' days are numbered. That's a good thing given the toll that burning coal and oil takes on our climate and health. But especially for the people of Beijing -- and Delhi and Tehran, among other cities currently choking on dirty emissions -- that final goodbye can't come soon enough.

The images are hard to miss and harder to ignore: women, men, children and even pets enveloped in an almost tangible haze, forced to wear face masks as they go about their daily lives -- bicycling, shopping, getting married.

The statistics are equally alarming: More than 4,000 Chinese die daily from air pollution. And that figure may even be an underestimate, as pollution levels have risen since researchers crunched those numbers earlier this year.

A tragedy is indeed unfolding, and threatening to escalate.

On multiple days this December, the air in Beijing measured at least 20 times dirtier than what the World Health Organization deems safe to breathe. The concentration of PM2.5 -- the tiny air particles that pose the greatest health risks -- reportedly reached 647 micrograms per cubic meter near Tiananmen Square on Christmas morning. This Tuesday, parts of Beijing again registered counts above 500. The WHO sets their limit of exposure at no more than 25 micrograms per cubic meter over a 24-hour period.

Experts warn that the situation will likely worsen in the weeks ahead.

"We're just getting into high season," said John Groopman, an environmental health expert at Johns Hopkins Bloomberg School of Public Health, as he prepped for a trip to China this week to research the issue. Cold weather, he explained, can trap polluted air near the surface of the earth. Meanwhile, more pollution is generally created during winter months due to increased heating, which is mostly supplied in China by burning coal.

The poster child for air pollution troubles, China also offers a cautionary tale for other parts of the world.

Pollution is currently soaring in parts of Iran and Italy, for example, where schools, vehicles, football matches and even pizza ovens have shut down in efforts to clean up the toxic air. Groopman suggests that India, whose rapidly growing population is even more dependent on coal than China, may be in the worst shape of all. A study released in February found that 660 million Indians lose an average of 3.2 years of life due to air pollution exposure.

Bad air from Asia can also travel overseas, contributing to the mercury and other pollution plaguing the U.S. West Coast.

Overall, according to the WHO, bad air causes the premature deaths of more than 7 million people every year. And the list of air pollution's effects is well-known and staggering: heart disease, lung disease, cognitive problems, obesity and even increased crime rates.

"It's really quite obvious that no one should be breathing this," said Groopman.

Of course, the key culprits behind this air pollution, coal-fired power plants, are also the leading emitters of greenhouse gases. China's share of global climate-changing carbon dioxide emissions is around 29 percent. Ironically enough, Beijing's first air quality "red alert" -- declared when authorities predict PM2.5 counts will surpass 300 for three consecutive days -- came just as world delegates met in Paris for the United Nations climate change summit in early December. The coincidence garnered significant press.

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There, with air pollution under the spotlight, nearly 200 nations pledged to wean themselves off climate-disrupting fossil fuels.

"Making clear what's at stake for the health of people and their children is ultimately, to me, one of the most powerful arguments we can make for a call to action [on climate change],” Dr. Aaron Bernstein, a pediatrician and environmental health expert at Harvard University, said during a Dec. 16 panel on climate change and health.

While climate change poses multiple threats to public health, including increased risks of infectious diseases and deadly heat waves, air pollution is among the most direct and obvious in its connections. Not only is it a consequence of burning fossil fuels, air pollution can also be exacerbated by a changing climate -- from more frequent and larger wildfires releasing smoke to warmer temperatures producing more ozone smog.

"Because we're doing something about climate change,” added Bernstein, "we actually stand at an entry point to perhaps the greatest public health intervention ever."