Acid rain is a serious environmental problem around the world, particularly affecting Asia, Europe, and large parts of the U.S. and Canada. The acidic pollutants such as SO₂ and NO_x are emitted into the environment by combusting fossil fuels.

Most of the sulfur in any fuel combines with oxygen and forms SO₂ in the combustion chamber. This SO_2, when emitted into the atmosphere, slowly oxidizes to SO₃. SO₃ is readily soluble in water in the clouds and forms H₂SO₄ (sulfuric acid).

$${\text{S + O}}_{\text{2}}{\text{ → SO}}_{\text{2}}{\text{ + 1/2 O}}_{\text{2}}{\text{ (in the atmosphere) → SO}}_{\text{3}}{\text{ +\hspace{0.17em}H}}_{\text{2}}{\text{O → H}}_{\text{2}}{\text{SO}}_{\text{4}}\text{ (sulfuric acid)}$$

Most of the NO_x that is emitted is in the form of NO. This NO is oxidized in the atmosphere to NO₂. NO₂ is soluble in water and forms HNO₃ (nitric acid).

$${\text{NO + 1/2 O}}_{\text{2}}{\text{ (in the atmosphere) → NO}}_{\text{2}}{\text{ +\hspace{0.17em}H}}_{\text{2}}{\text{O → HNO}}_{\text{3}}\text{ (nitric acid) }$$

Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3. By the 2020's most locations of the US have precipitation with pH of 5.0-5.5. 

Below is a video demonstration that replicates the effect of acid rain on plant life. In this video, beans are placed in: a) water, b) slightly acidic water and c) acidic water, and their growth is observed over a period of three days. Please watch the following 5:35 video:

Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. However, the pollutants that cause acid rain also damage human health.

• Effects of Sulfur Dioxide (SO₂): These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled deep into people's lungs. Fine particles can also penetrate indoors. Many scientific studies have identified a relationship between elevated levels of fine particles and increased illness and premature death from heart and lung disorders, such as asthma and bronchitis.
• Effects of Nitrogen Oxide (NO_x): Decrease in nitrogen oxide emissions are also expected to have a beneficial impact on human health by reducing the nitrogen oxides available to react with volatile organic compounds and form ozone. Ozone impacts on human health include a number of morbidity and mortality risks associated with lung inflammation, including asthma and emphysema.

Text description of the Annual Wet Sulfate Deposition image.

The image is a comparison of two maps of the United States showing annual wet sulfate (SO₄²⁻) deposition over two different time periods, 1989-1991 and 2020-2022. Each map displays the continental U.S. with color gradients representing the levels of sulfate deposition.

On the left, the map from 1989-1991 shows high levels of sulfate deposition concentrated mostly in the eastern and central regions, especially pronounced in the Ohio Valley and surrounding areas, depicted in dark red and orange, indicating higher levels. The western regions are shown in lighter greens and yellows, indicating lower levels of deposition.

On the right, the map from 2020-2022 has a more uniform teal color across the entire country, suggesting significantly reduced sulfate deposition levels compared to the earlier period. This illustrates a substantial improvement in air quality over time.

Credit: Acid Rain Program Results. NADP, PRISM, USEPA. Accessed May 29, 2026.

Text description of the Annual Sulfur Dioxide Emissions image.

The image is a bar and line graph titled "Annual Sulfur Dioxide Emissions, 1990–2020." It displays data on sulfur dioxide emissions and gross electricity generation in the United States over the period of 1990 to 2020. The vertical axis on the left measures sulfur dioxide emissions in million short tons, ranging from 0 to 17.5, while the right vertical axis measures gross generation in billion megawatt-hours (MWh), ranging from 0 to 3.5. The bars in blue represent the sulfur dioxide emissions for each year, starting at 15.73 million short tons in 1990 and decreasing to 0.79 million short tons in 2020. The green line indicates gross generation, which shows a generally increasing trend from 1990 until around 2008, then slightly decreasing towards 2020. The graph includes a note that data for sulfur dioxide emissions from 1991 to 1994 are not available.

Data Table for the Annual Sulfur Dioxide Emissions, 1990-2020

Year	Sulfur Dioxide (million short tons)	Gross Generation (Billion MWh)
1990	15.73
1991
1992
1993
1994
1995	11.83
1996	12.51
1997	12.94	2.25
1998	13.09	2.34
1999	12.45	2.39
2000	11.20	2.49
2001	10.64	2.46
2002	10.20	2.48
2003	10.59	2.53
2004	10.26	2.58
2005	10.22	2.74
2006	9.39	2.72
2007	8.93	2.83
2008	7.62	2.78
2009	5.82	2.65
2010	5.17	2.80
2011	4.55	2.73
2012	3.32	2.71
2013	3.24	2.69
2014	3.16	2.70
2015	2.22	2.66
2016	1.49	2.59
2017	1.34	2.49
2018	1.26	2.61
2019	0.97	2.53
2020	0.79	2.38

Credit: Whatever Happened to Acid Rain?. American Council on Science and Health. Accessed May 29, 2026.

The earth’s ozone layer protects life from harmful radiation from the sun. Ozone is comprised of three oxygen molecules bonded together. Without this layer of protection, too much UV light would hit the earth’s surface, resulting in damage to crops and increased skin cancer and cataract rates in humans.

In the 1970’s scientists discovered that some chemicals such as: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, and methyl chloroform were depleting the ozone layer. These chemicals were often found in refrigerants, fire suppressants, foam insulation and aerosols.

In 1985 the Vienna Convention on the protection of the ozone layer formalized international cooperation to find a solution. The Montreal Protocol on Substances that Deplete the Ozone Layer was an international agreement signed in 1987 which phased out the use of these ozone depleting chemicals on a global scale. The positive impact of this international agreement is why many of you may have never heard of the hole in the ozone layer. For earlier generations (those born in 1970-1990s) would have grown up learning about this in environmental issue beginning in elementary school. Thanks to the Montreal Protocol and international cooperation, the hole in the ozone layer is on track to be fully recovered by 2040.

2025 Ozone Hole Update (0:48)

Note: The video has music in the background but no spoken words.

Lead was added to gasoline in the 1920’s to reduce “knocking” and improved fuel efficiency.   But lead proved to be a toxic pollutant, having a major effect on the cognitive development of children.   Many countries began phasing out the use of leaded gasoline in the 1970s, which is why you still find signs indicating “unleaded gasoline” at the gasoline pump today.   Japan was the first country to entirely ban leaded gasoline in 1986. The US and Canada banned it in 1996.

For more information on the phase of Lead in gasoline: Please read this article.

• How the world eliminated lead from gasoline - Our World in Data

Did you know, Algeria was the last country to finally ban leaded gasoline, but it wasn’t until 2021! 

Use the slider at the bottom of the Map to see how and when lead was phased out of on road fuels!