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Potential risks of Stratospheric Aerosol Injection (SAI): human health impacts

Summary​

Stratospheric Aerosol Injection (SAI) poses potential human health risks through two primary pathways: ozone layer depletion and sulfur exposure. Ozone damage could increase UV radiation exposure, potentially raising risks of skin cancer, cataracts, and immune system impacts, though vitamin D production might see benefits. Sulfur dioxide (SO₂) used in SAI could theoretically cause respiratory and cardiovascular issues, though stratospheric injection likely reduces direct inhalation risks compared to ground-level pollution. These risks must be balanced against potential health benefits from climate change mitigation, including reduced heatwaves, infectious diseases, and surface air pollution. Marine fuel sulfur regulations demonstrate the health significance of atmospheric sulfur levels.

Stratospheric Aerosol Injection (SAI) can impact human health through a number of pathways. Aerosols can impact human health when they make their way down to the lower atmosphere, where they can be inhaled or absorbed by the human body. SAI can also impact human health through its potential to damage the ozone layer, which protects the human body from harmful ultraviolet (UV) radiation. However, any discussion regarding the human health impacts of SAI also needs to take into account the potential health benefits given by slowing down or halting climate change 1. Slowing the rise of global temperatures will reduce the spread of many infectious diseases, the frequency and intensity of heatwaves, and the concentrations of low-altitude pollution, which are linked to higher temperatures 1.

Effects of ozone depletion​

SAI can damage the Earth's ozone layer through a number of pathways. Ozone is vital for protecting the Earth's surface and life from harmful ultraviolet (UV) radiation, and it is well understood that SAI poses a risk to the effectiveness of the ozone layer. The effects of UV radiation on human health are well understood, and it is known that high quantities of UV radiation can lead to increased rates of skin cancer, cataracts, and other health problems 2. Exposure to UV radiation can also cause damage to the human immune system, but it should also be noted that UV-B radiation is the main way in which vitamin D is synthesized in the human body, which carries a number of health benefits 2. The degree to which SAI deployment will damage human health via ozone depletion is not well understood, and estimates to predict premature mortality carry high degrees of uncertainty 1. However, comparisons have been made to the eruption of Mt. Pinatubo in 1991, which reduced global temperatures by an average of 0.9°F while destroying 2.5% of global ozone 3. Estimates suggest that in order to achieve the same levels of global temperature reduction, SAI would require five times less sulfur dioxide than was emitted by Mt. Pinatubo 3.

Sulfur poisoning​

Sulfate aerosols are often touted as the main candidate for SAI due to their well-studied ability to scatter light, as well as their relative cost-effectiveness. SAI using sulfate aerosols is achieved by injecting sulfur dioxide (SO₂) into the atmosphere where it reacts with water vapor to form sulfuric acid (H₂SO₄). Sulfur dioxide is a common toxin released in the combustion of fossil fuels and is therefore present in most atmospheres where there is human activity. When inhaled, sulfur dioxide can cause respiratory, cardiovascular, and neurological issues and has been known to cause excess deaths and hospitalization after exposure to high atmospheric concentrations for long periods 4. It is for this reason that the International Maritime Organization introduced new fuel limits to sulfur content in 2020, which they estimated would save over 570,000 premature deaths in the 5 years following the change 5. However, these concerns are in relation to sulfur dioxide concentrations in the lower atmosphere, where they can be breathed in by humans. A key difference between SAI and natural aerosols is that SAI injects sulfur dioxide into the lower stratosphere (8 – 18 km or 5 – 11 miles above ground level), which would not lead to health complications via sulfur dioxide inhalation. Additionally, the large majority of aerosols released into the stratosphere would mostly return to the surface via wet deposition, meaning that they would not be inhaled by humans 1.

Sources​

Footnotes​

  1. National Academies of Sciences, Engineering, and Medicine. (2021). Reflecting sunlight: Recommendations for solar geoengineering research and research governance. The National Academies Press. https://doi.org/10.17226/25762 ↩ ↩2 ↩3 ↩4

  2. Solomon, S. (2008). Effects of ozone depletion and UV-B radiation on humans and the environment. Environmental Health Perspectives, 116(12), A514-A515. https://doi.org/10.1289/ehp.116-a514 ↩ ↩2

  3. Crutzen, P. J. (2006). Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma? Climatic Change, 77(3), 211-220. https://doi.org/10.1007/s10584-006-9101-y ↩ ↩2

  4. Khalaf, E., Mohammadi, M., Sulistiyani, S., Ramírez-Coronel, A., Kiani, F., Jalil, A., Almulla, A., Asban, P., Farhadi, M. & Derikondi, M. (2024). Effects of sulfur dioxide inhalation on human health: a review. Reviews on Environmental Health, 39(2), 331-337. https://doi.org/10.1515/reveh-2022-0237 ↩

  5. International Maritime Organization. (2020). Marine Environment Protection Committee 70th session annex 1. Retrieved from https://www.imo.org/en/OurWork/Environment/Pages/MEPC.aspx ↩