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What Is Acid Rain?

Medically Reviewed by Dan Brennan, MD on November 21, 2022

Acid rain may not be the hot-button issue that it once was, at least in the U.S. The 1990 Clean Air Act Amendments helped greatly lower the levels of pollutants that turn rain more acidic. But acid rain still falls across the globe. And some of the gasses that cause acid rain also hasten climate change.

What Is Acid Rain?

It’s when water and oxygen in normal precipitation mix with compounds such as sulfur dioxide and nitrogen oxides in the atmosphere. The chemical reaction turns clean rain -- which is naturally acidic -- dangerously so.

Scientists measure acidity on the pH scale from 0 to 14. Water with a pH of 7 is neutral. Battery acid is most acidic with a pH of 0. Liquid drain cleaner falls on the other end of the scale, called basic, with a pH of 14.

Clean rain normally falls about 5.6 on the pH scale; acid rain falls below 4.5 pH.

Acid rain comes in two forms:

Wet. The compounds turn into acids in the atmosphere and fall down as rain, sleet, snow, fog, or hail.

Dry. This is when compounds produce acids and float down to the ground as dust and gas. Future rains can spread them even more. You can also breathe it in.

What Causes Acid Rain?

Much of the acids originate from power plants that burn coal, oil, and other fossil fuel to generate electricity, as well as from exhaust from gasoline-powered vehicles.

Sulfur dioxide and nitrogen oxides can travel high into the atmosphere. When the emissions combine with precipitation, they turn into harmful sulfuric and nitric acids. Sometimes, the earth releases sulfur dioxide and nitrogen oxides naturally, like when a volcano or geyser erupts.

The acids produced in the acid rain can fall near the original source of pollution. Winds can also blow contaminated dust in the atmosphere even farther and it can fall there, blanketing more areas.

Can Acid Rain Hurt You?

Yes. But the danger doesn’t come from swimming in a lake containing acid rain or getting wet from raindrops. The harm comes from breathing in particles from acid rain.

If you’re exposed to high concentrations of nitric and sulfuric acid -- especially over time -- it can cause these problems:

Irritation to eyes, skin, and mucous membranes can come from contact with one or both acids.

Fluid in lungs, or pulmonary edema, can happen if you breathe in nitric acid.

Dental erosion. Both acids can wear down the enamel on your teeth.

The compounds released from burning fossil fuels (before they turn into acids) can cause:

Respiratory illnesses like chronic bronchitis, pneumonia, and asthma. (Acid rain chemicals can make existing respiratory ailments worse, too.)

Cardiovascular problems, such as worsening existing heart disease.

Lower birth weight, which might affect a child’s growth and development.

Lung cancer. Pollution can cause cell mutations that can become cancerous.

How Does Acid Rain Affect the Environment?

Acid rain can not only harm human health, but it’s also damaging to our lakes, plants, and buildings.

Soil. Generally, limestone (a base) in the ground helps neutralize the acid. But acid rain can affect soil that doesn’t contain much limestone. Also, the chemicals can trickle into the water below to the groundwater. That can acidify major waterways, too.

Surface water. Acid rain can fall into a body of water like a lake or the ocean. It may build up more in waters that don’t move as much, such as ponds. Over time, it can change the water’s pH balance.

Animals. Acid rain can affect plants and animals that rely on the polluted water. That in turn affects bigger animals that feed on them. The result: Our foods may be contaminated.

Plants and forests. Acid rain can destroy nutrients like calcium and magnesium that keep trees healthy. It releases aluminum, which makes it tougher for trees to get water from the ground. Trees located at higher elevations are more prone to acidic clouds and fog as well.

Buildings. Chemicals from acid rain can break down stone and metal on structures. This can wear down surfaces and fade finishes.

Where Does Acid Rain Fall?

Wind can carry acid rain dust around the globe far from the source of the pollution. The U.S. tracks the chemicals in acid rain. The Northeastern region is the most acidic because it has many power and industrial plants, people, and cities. Many plants in the Midwest release harmful chemicals that also blow to the Northeast, increasing the acidity. Also, the soil in the Northeast is less likely to be able to neutralize the acid rain.

Serious acid rain remains a problem in parts of China and elsewhere in Asia that are rapidly industrializing.

What’s the Link Between Acid Rain and Climate Change?

Both stem from pollution created by humans. Even though acid rain is a regional problem, it can eventually alter the chemical makeup of soil and water around the world.

Some compounds that cause acid rain, including nitrous oxide, are greenhouse gasses like carbon dioxide and methane that trap heat in the atmosphere. That contributes to global warming. In turn, the higher average temperatures are melting glaciers and setting off more and more extreme weather changes that are a sign of climate change.

Global warming can make acid rain worse because it speeds up the chemical reactions that produce acid rain.

What Can We Do About Acid Rain?

Many coal plants -- once a major source of acid rain -- in the U.S. are shutting down and more will be retired. There are no plans for new large-scale coal plants in the country, though other countries still build them.

Individual actions also can help prevent acid rain. You can:

Try battery-operated tools. Switch from gas-powered tools to ones with rechargeable batteries.

Drive an eco-friendly vehicle. Electric and hydrogen-powered cars don’t produce air pollution, and hybrid vehicles burn less fuel than gas-powered ones. They can reduce fuel consumption and pollution. Low-emissions vehicles give off a lot less compared to other cars.

Conserve energy. Using less energy means producing less energy. Turn off electronics when you don’t use them, lower the heat when you’re not using it, and limit air conditioning. You can also replace incandescent light bulbs with compact fluorescent light bulbs or choose Energy Star appliances -- they use less energy.

Opt for renewable energy. Add solar panels to your home. They create less pollution than fossil fuels. If you live in a deregulated state, you can choose an electricity provider that uses renewable energy instead of one that uses fossil fuels.

Drive less. Exhaust from cars, trucks, and buses spew nitrogen oxides and sulfur dioxide. Consider walking or biking somewhere instead of driving when you can, or use public transportation.

Burn better. If you have a fire, keep it small and only burn wood -- not other waste, especially in cities that tend to have higher air pollution. Wood-burning fireplaces should only burn wood, too.

Show Sources

SOURCES:

U.S. Environmental Protection Agency: “Acid Rain Program,” “Acid Rain Program Results,” “Basic Information about NO2,” “Effects of Acid Rain,” “Learning About Acid Rain,” “Nitrogen Oxides (NOx) Control Regulations,” “Overview of Greenhouse Gases,” “What Can You Do?” “What Causes Acid Rain?” “What Is Being Done?” “Why is Acid Rain Harmful?”

New Hampshire Department of Environmental Services: “Environmental Fact Sheet – Acid Rain.” 

Elmhurst College: “Acid Rain Transport.”

CDC: “Nitric Acid,” “Sulfuric Acid.”

National Park Service: “Sulfur Dioxide Effects on Health.”

American Lung Association: “Nitrogen Dioxide.”

Annals of Global Health: “Prenatal Exposure to Nitrogen Oxides and its Association with Birth Weight in a Cohort of Mexican Newborns from Morelos, Mexico.”

Cancer Research UK: “Scientists Reveal How Air Pollution Can Cause Lung Cancer in People Who Have Never Smoked.”

National Science Foundation: “Acid Rain: Scourge of the Past or Trend of the Present?”

International Society for the Study of Lung Cancer: “Air Pollution and Lung Cancer - The IASLC Position Statement.”

Thorax: “Air Pollution Affects Lung Cancer Survival.” 

United States Geological Survey: “Acid Rain and Water,” “National Atmospheric Deposition Program (NADP),” “pH and Water.”

American Ground Water Trust: “Acid Rain and Ground Water pH.”

National Resources Defense Council: “Global Warming 101.”

United Nations: “The Links Between Climate Change and Acid Rain Policies.”

Syracuse University: “Rules to Cut Carbon Emissions Also Reduce Other Air Pollutants.”

NASA: “What’s the Difference Between Climate Change and Global Warming?”

U.S. Energy Information Administration: “Of the Operating U.S. Coal-Fired Power Plants, 28% Plan to Retire by 2035.”

Minnesota Pollution Control Agency: “What You Can Do About Air Pollution.” 

U.S. Department of Energy: “Alternative Fuels Data Center,” “Buying Clean Electricity,” “DOE Report Finds Hundreds of Retiring Coal Plant Sites Could Convert to Nuclear,” “Fossil.” 

California Air Resources Board: “All Californians Can Make a Difference.”

Energy Star: “ENERGY STAR Impacts.”

Acid Deposition Monitoring Network in East Asia: “CHINA Policies and Practices Concerning Acid Deposition.”

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