Just over 40 years ago the U.S. government passed a federal law regulating atmospheric emissions – the Clean Air Act. Without it, the air we breathe today would be very different. Rather than stretching up into a clear blue skyline, U.S. cities would be polluted with smog, limiting visibility and posing a public health risk to everyone exposed to it.

Air Pollution in U.S. Cities

To determine what pollution concentration levels would be like today if the Clean Air Act was not enforced and no action were taken to reduce air pollution levels, we used an equation.* The formula converted the concentration of particles in the air to visibility in miles and adjusted for impurity levels from China’s most polluted city, Xingtai, to American cities (according to population).

Take a look at our results in the images below. You’ll be very thankful that we adopted the Clean Air Act.

Alternate Reality: U.S. Cities Without the Clean Air Act

Chicago: The view from the Skydeck of the Willis Tower would not be so spectacular if the skyline were shrouded in haze.

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Dallas: The 360-degree view from Dallas’ Reunion Tower would give you a panoramic look at 50 shades of gray.

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Houston: The tree-lined streets of Houston would be gritty, grimy, and gray.

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Los Angeles: The home of Tinsel Town would not be so glitteringly fabulous if it were smothered in smog.

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New York: The subdued green of the Statue of Liberty on Liberty Island in New York Harbor would be submerged in a sea of smog.

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Philadelphia: You wouldn’t be able to read the time on City Hall’s clock tower or see much from the observation deck if Philly were swathed in smog.

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Phoenix: Yep, if the heat in Phoenix didn’t kill you first, the air pollutants might …

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San Antonio: Rather than marking the fight for independence from Mexico, The Alamo and its residents would be fighting a battle for survival in a heavily polluted city.

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San Jose: All the technology in the world would not improve the outlook for San Jose if it were entombed in smog.

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Causes & Effects of Atmospheric Pollution

Air pollution occurs when particulate matter, biological agents, or other harmful pollutants are introduced into the atmosphere, posing both an environmental and human health risk. The World Health Organization (WHO) considers air pollution to be the “contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere.”

Causes of Air Pollution

Pollutants can enter the atmosphere from many different sources, including smoke from household fires, wildfires, industrial facilities, and motor vehicle exhaust fumes. Fossil fuels – such as coal, gasoline, oil, and natural gas – are burned to generate energy. That energy helps us stay warm, keeps factories in production so they can manufacture the stuff we consume, powers our gadgets, and runs our motor vehicles; it also creates carbon dioxide (CO2), which is released into the atmosphere in the process.

Carbon dioxide is not only an abundant greenhouse gas that is a key culprit in global warming and climate change, but it also gives us a good indication of how much fossil fuel is being burned as well as a clearer picture of the extent of pollutants being emitted into the atmosphere as a consequence of this.

These atmospheric pollutants – including nitrogen dioxide and sulfur dioxide, which can cause acid rain – can damage natural ecosystems as well as the built environment. They can harm other living organisms, including both animals and plants (food crops too), and can cause allergies and serious diseases in humans, which can even be fatal.

How Does Air Pollution Affect Our Health?

According to the EPA, when we are exposed to high levels of hazardous air pollutants for an extended period of time, we face a greater risk of suffering serious health effects, including respiratory diseases, neurological disorders, reduced fertility, damage to our immune system, and increased risks of getting cancer. As air pollution in urban areas increases, so does the potential risk of heart disease, stroke, lung cancer, and chronic and acute respiratory diseases, such as asthma, for residents of these cities.

Furthermore, some toxins that are emitted into the atmosphere are deposited onto soils, freshwater bodies, and the surface of the oceans, where they may be absorbed by plants or ingested by fish and other wildlife. Mercury, for example, is a highly toxic air pollutant that persists in the environment in the form of methylmercury, and as it is stored in the body fat of animals, it bioaccumulates. This makes it more and more concentrated further up the food chain.

Consequently, top predators, such as East Greenland polar bears that live hundreds of miles from the source of the pollution, have the highest levels of mercury in their bloodstream ever measured in arctic mammals. As top predators, humans are also at risk when consuming contaminated food items, such as tuna – a predatory fish high up the marine food chain that is known to contain significant levels of mercury.

Methylmercury is a potent neurotoxin that can cause serious health effects to both animals and humans exposed to it at high levels. The possible health effects? Impaired speech, vision, hearing, and walking; tingling sensations in the hands, feet, and around the mouth; muscle weakness and loss of coordination. Fetal exposure to methylmercury while in the mother’s womb can result in developmental, cognitive, and motor coordination problems in infants and children.

Worst Case Scenario: Xingtai, China

The World Health Organization considers atmospheric pollution to be the greatest environmental threat to human health, resulting in more than 3 million deaths worldwide every year. A large percentage of those fatalities are from China, which has arguably the worst air pollution record in the world.

This can largely be attributed to its high population and thriving industrial sector, with government policy historically focusing more heavily on economic growth than environmental health. According to the latest annual estimates put forward by the World Health Organization, 299,400 people die prematurely in China each year due to poor air quality. A recent air quality assessment report of major Chinese cities shows that out of the five cities studied, Beijing, Chengdu, and Shenyang fair the worst, while Guangzhou has the best air quality, followed by Shanghai.

The concentration of atmospheric particulate matter of a size that is able to penetrate the human lungs and blood stream (PM2.5 micrometers) is 10 times greater in Beijing compared with that of New York, and it threatens the health of 22 million citizens living in the Chinese capital. However, Beijing is not the most polluted city in China; this honor is bestowed on the industrial city of Xingtai, with a PM2.5 reading of 128 micrograms per cubic meter – 110 micrograms higher than the 10 micrograms per cubic meter of PM2.5 recommended by WHO as a level for safe air.

Impact of the Clean Air Act

The U.S. can largely thank the Clean Air Act, which was originally passed in 1973 and amended in 1990, for its clean air record compared with that of China. Without these regulations, the air in U.S. cities could be as bad, if not worse, than that of Chinese cities.

The Clean Air Act was passed to reduce the impacts of air pollution on both environmental and human health. It requires the U.S. Environmental Protection Agency to regulate and monitor atmospheric emissions and toxic pollutants that pose a risk to public health.

According to the Union of Concerned Scientists, since the Clean Air Act was passed over 40 years ago, pollution levels have been cut dramatically, preventing the premature deaths of over 400,000 people as well as preventing hundreds of millions of people from contracting diseases related to air pollution. Some of the key air quality improvements since the Clean Air Act was implemented include the following:

  • Surface ozone levels have dropped by over 25% since 1980.

  • Mercury emissions have been reduced by 45% since 1990.

  • The two key pollutants responsible for acid rain – nitrogen dioxide and sulfur dioxide – have been reduced by 46% and 71% respectively, since 1980.

  • Chemicals that are responsible for the hole in the ozone layer, such as CFCs, have been phased out and are no longer produced or used extensively.

  • Lead in gasoline has been substantially reduced, resulting in lead air pollutants being cut by 92% since 1980.

Thankfully, the U.S. has been proactive in its approach to curbing atmospheric pollution. As we can see in the slider images of our cities above, rather than being surrounded by clean air and blue skies, we could be experiencing a very different doom-and-gloom scenario if the Clean Air Act was not introduced back in 1970.


This project was created to illustrate the importance of the Clean Air Act to our everyday life, our health, and the environment. We juxtaposed two images of each skyline: One image is what the city looks like today, and the other image is a modified rendering of the skyline that shows what visibility could look like if there were no Clean Air Act due to excessive smog and pollution.

The first step was to determine which city to use as a basis for comparison. We selected the Chinese industrial city of Xingtai because it is one of the world's most polluted cities. The goal was to have an image showing what the cities in our study would look like if they had similar pollution levels.

Next, we used an equation* that translates the concentration of particles in the air to visibility in miles. The book “Energy Development, West Central North Dakota” published the equation. We adjusted visibility in each city to account for population.

Finally, we used a graphic designer to apply the visibility effects (based on the equation) to our images.

The U.S. cities with the largest populations were chosen.


Adj. Max. Daily PM2.5 for Population = (US Pop. / Xingtai Pop.) * Max Daily PM2.5 for Xingtai

VL = (A * 10^3)/G’

VL = Equivalent visual range.

A = 0.75 Adjusted for miles

G = Micrograms per cubic meter.


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