While not as thick as the Great Smog of 1952, smog remains an issue in London. Shown here is compromised visibility from a smog episode in April 2011 (Matt Dunham/AP)

Sixty years ago this month (December), London was enveloped under a toxic mix of dense fog and sooty black smoke for four days. This episode of polluted air is among the deadliest environmental disasters in recorded history.

The event became known as the Great Smog of 1952. Over 4,000 more people died than usual for that time of year with an estimated 8,000 more fatalities in the following weeks from exposure to the noxious air pollution.

While not as deadly – and not as well-known - hundreds of deaths have been attributed directly to episodes of severe smog in the U.S. over the 60 intervening years - including three killer smog events in New York City.

The setup for the deadly London smog episode involved a stagnant very cold high pressure region that settled over the city December 5, 1962. Along with the usual factory emissions of coal smoke and soot, increasing burning of coal for home heating added to smoke levels pouring from chimneys. The meteorological conditions were favorable for fog formation and characterized by little wind to disperse smoke horizontally. In addition, an inversion above the surface trapped air from rising vertically. As such, the combination of smoke and fog inevitably led to an increasingly dense layer of smog.

Link: Pictures of 1952 London smog (BBC)

The choking smog enveloped the city for four days. In some places, even daytime visibility was so limited you couldn’t see your own feet. With near zero visibility, roads were littered with abandoned cars. Midday concerts and social events were cancelled due to total darkness and virtually all commerce was shut down.

People died in their homes and in the hospitals, mainly older audlt with existing cardio-respiratory illnesses. Amazingly it was reported that no one noticed the rising death toll: “There were bodies lying around the streets … One of the first indications that things were happening is that undertakers were running out of coffins, florists were running out of flowers,” writes Jennifer Rosenberg of About.com.

On December 9, wind swept in unexpectedly and the killer smog vanished as quickly as it had arrived.

Before the Great Smog of 1952, Londoners tended to accept pollution as part of life of the industrial age. Afterward, those who had survived no longer spoke of London’s romantic pea-soup fog. Rather, once the smog lifted they increasingly fought to limit the poisons emanated as side effects of industrialization. Environmental research subsequently increased awareness of the relationships between air quality and health. This led to the British Clean Air Act 1956.

There has not been anything close to the scale of Great Smog in the decades following, but experts say the present levels of air pollution, although not as visible, may be just as much of a threat. A 2010 report to the UK Parliament found that in 2008 4,000 people died in London from air pollution (the same number from the Great Smog of 1952) and 30,000 died across the whole of the UK.

The culprit now, however, is not just burning of coal and the resulting smoke/soot, but also vehicle and industrial fossil fuel emissions. These emissions react chemically in the presence of sunlight to form ground-level ozone, or photochemical smog.

Like the smoke/soot smog that enveloped London, both soot and photochemical smog have been deadly in the U.S. In November 1953, smog killed between 170 and 260 people in NYC; 10 years later it killed 200; and in 1966 it killed 169 (sources Gothamist, New York Times).

Smog over New York City in 2006. (NASA)

These “horrors of New York’s environmental past” evoked congressional hearings to study the nature and sources of the smog and options for mitigating the hazards. Still, in 2006, the Environmental Protection Agency (EPA) declared, “that 68 out of every million New Yorkers are at risk for getting cancer just from breathing the air”.

Given that the Washington, D.C. metro region is known for some of the worst traffic in the country, it should not be surprising that the area is ranked also among the smoggiest metropolitan areas in the country.

The 2012 State of the Air Report from American Lung Association ranked Washington, D.C. 13th among the most polluted cities for ozone, and 22nd for short-term particle pollution (e.g. from soot). A 2011 report from Penn Environment ranked Washington, D.C. and Baltimore third smoggiest among major metropolitan areas.

I can personally attest to the elevated pollution levels locally with regard to my vulnerability to asthma attacks.

Although air pollution is certainly present both locally and elsewhere across the country, reforms under EPA’s Clean Air Act have resulted in demonstrable improvements in air quality in the last few decades. EPA’s most recent Air Trends report shows the national average ground level ozone concentration decreased 28 percent from 1980-2010 and large and small particle pollution also decreased substantially.

Despite these gains, Charles Connor, president of the American Lung Association said last spring “....America’s air quality standards are woefully outdated, and unhealthy levels of air pollution still exist across the nation, putting the health of millions of Americans at stake.”

In September 2011, facing stiff Republican opposition, President Obama decided to delay tightening the nation’s smog standards. On the other hand, just last week the Post’s Juliet Eilperin reported that EPA was tightening soot standards by 20 percent.

Jason Samenow contributed to this post