What Is Acid Rain?

Medically Reviewed by Jabeen Begum, MD on November 15, 2023
7 min read

Acid rain is a broad term for any precipitation that has more acid in it than normal. For example, acid rain may have sulfuric acid or nitric acid. Precipitation includes not only rain but also fog, hail, sleet, and snow. Acid rain can hurt the environment and your health. Some of the gasses that cause acid rain can also speed up climate change.

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.

You might not hear about acid rain as much as you used to. In the U.S., the 1990 Clean Air Act Amendments helped lower the levels of pollutants that cause rain to become more acidic. But acid rain still falls across the globe.

Acid rain pH

The pH scale ranges from 0 to 14. It tells you how acidic something is. Seven, in the middle, is neutral like pure water. A pH less than 7 means a substance is acidic. A pH over 7 means something is basic (alkaline) and not acidic. To get a better sense of what this means, it may help to know the pH levels of everyday items or other substances:

  • 0 (very acidic): Battery acid
  • 1: Stomach acid
  • 2: Lemon juice or vinegar
  • 3: Orange juice or soda
  • 4: Tomato juice
  • 5: Black coffee
  • 6: Urine or milk
  • 7: Pure water
  • 8: Seawater
  • 9: Baking soda
  • 10: The Great Salt Lake
  • 11: Ammonia
  • 12: Soapy water
  • 13: Bleach
  • 14 (very basic or alkaline): Liquid drain cleaner

Normally, rain is a little acidic, with a pH of about 5.6. The pH of acid rain usually falls between 4.2 and 4.4, about the same as that of tomato juice.

Acid rain forms when water and oxygen in normal precipitation mix with chemicals such as sulfur dioxide and nitrogen oxides. The chemical reaction turns normal rain more acidic.

Primary sources of sulfur dioxide and nitrogen oxides in the atmosphere are:

  • Power plants. Two-thirds of sulfur dioxide and a quarter of nitrogen oxides in the atmosphere come from electric power generators.
  • Vehicles and heavy equipment.
  • Other industries. Examples include manufacturing and oil refineries.

Once these particles are released into the air, wind and air currents sweep them up. These particles then mix with water, oxygen, and other chemicals to create sulfuric acid and nitric acids.

These acids usually mix with water and fall to the ground as precipitation. Sometimes, when there isn’t enough moisture, the particles fall to the ground and land on surfaces like bodies of water, buildings, trees, and plants. When precipitation eventually comes, it can wash the particles into the soil and bodies of water.

Acid rain can harm your health. But the danger doesn’t come from swimming in a lake containing acid rain or getting wet from acidic 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 may cause these problems:

  • Irritation to the eyes, skin, and mucous membranescan come from contact with one or both acids.
  • Fluid in the lungs,or pulmonary edema, can happen if you breathe in nitric acid.
  • Dental erosion, if acids wear down the enamel on your teeth.

The sulfate and nitrate particles that turn rain acidic also may cause or worsen:

  • Respiratory illnesses, including chronic bronchitis, pneumonia, and asthma 
  • Cardiovascular problems, such as existing heart disease
  • Lower birth weight, which might affect a child’s growth and development
  • Lung cancer, if chemicals in the air you breathe cause cells in your lungs to turn cancerous

Acid rain not only harms human health but is also damaging to animals. Animals that live in aquatic environments, such as lakes, rivers, and streams, are most affected by acid rain. While not all animals are sensitive to acidity, creatures such as clams and snails need a pH of at least 6 to survive. The young of many species often don’t handle acidity as well as the adults. While an adult fish may be able to handle water with a lower pH, their eggs may not hatch.

The inability of the young to survive isn’t the only threat to species. Acid rain can also disrupt food chains. For example, while frogs can survive in water with a pH as low as 4, the mayflies they eat may not survive below a pH of 5.5. This can have a cascading effect, wherein even if a species can survive the acidity, they may still starve. This includes not just aquatic animals but also animals such as bears and wolves.

Other effects of acid rain on the environment include:

Bodies of water. Acid rain can increase the dangers of lakes, rivers, and streams more than you might expect. When acid rain falls to the ground and flows into the soil, it absorbs aluminum from the surrounding soil. This then flows into the lakes, rivers, and streams and can make the water toxic to the creatures that live there.

Plants and forests. Acid rain can destroy nutrients such as 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.

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 groundwater. That can acidify major waterways, too.

Visibility. Particles in the air, like other forms of pollution, can cause smog and haze. This not only gets in the way of your view but can also be dangerous to travel in.

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 is a problem in parts of China and elsewhere in Asia that are rapidly industrializing.

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

Some compounds that cause acid rain, including nitrous oxide, are greenhouse gases. Like carbon dioxide and methane, they trap heat in the atmosphere, which adds to global warming. In turn, higher-than-average temperatures can melt glaciers to set off more and more extreme weather changes that are a sign of climate change.

Global warming can also make acid rain worse by speeding up the chemical reactions that produce acid rain.

Because acid rain is caused by burning fossil fuels, the best way to prevent acid rain is to find other ways to get power.

The Environmental Protection Agency (EPA) has already started on this. The Acid Rain Program sets a cap on the amount of sulfur dioxide and nitrogen oxides that power plants can release into the air. It also offers incentives for reducing these emissions.

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-emission 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 burn only wood -- not other waste, especially in cities that tend to have higher air pollution. Wood-burning fireplaces should only burn wood, too.