A possible geoengineering method to mitigate the global warming aspect of climate change is the injection of aerosols into the lower stratosphere, closely mimicking the way large volcanic eruptions cool the climate. This method is called solar-radiation management (SRM) scheme or simply Solar Geoengineering (1). SRM has been suggested to be affordable and have high effectiveness compared with other geoengineering schemes that have been suggested to mitigate global warming (1). While sulphate aerosols are the most studied, it has been recently shown that aerosols with other compositions, aluminium oxide (alumina) and diamond, could be used to dramatically increase the amount of light scatter achieved on a per mass basis. Alumina particles formed after the alumina aerosol injection are more efficient scatterers and may have less severe technology-specific risks than sulfates. Thus, they are expected to be more efficient per unit mass for geoengineering applications (2).

Nevertheless, it has also been suggested that SRM has a low associated safety compared with other geoengineering schemes because of its possible effects on regional climate, stratospheric ozone, high-altitude tropospheric clouds, biological productivity (3), and global biodiversity (4). We want to add a possible severe effect on Global Mental Health that could be caused by using alumina as light scattering. The average residence time of a particle in the lower stratosphere is approximately 1-2 years (1,3). After eventual transport into the troposphere, alumina particles undergo relatively rapid mixing processes by weather events, turbulence, and cloudscale overturning. They are mostly removed from the atmosphere by dry deposition, sedimentation, or scavenging by clouds, finally polluting the environment (1).

Aluminium has often been regarded as not posing a significant health hazard if the human body burden of aluminium has increased. Nevertheless, epidemiological studies suggest that aluminium may not be as innocuous as was previously thought and that aluminium may actively promote the onset and progression of Alzheimer's disease. This condition is the most common form of dementia and may contribute to 60 –70 % of cases. In 2015, dementia affected 47 million people worldwide (or roughly 5% of the world's elderly population), a figure predicted to increase to 75 million in 2030 and 132 million by 2050. Recent reviews estimate that each year nearly 9.9 million people develop dementia globally; this figure translates into one new case every three seconds (5). Even prolonged exposure to low levels of aluminium leads to changes associated with brain ageing and neurodegeneration (6).

Furthermore, aluminium has been included among 200 neurotoxic chemicals that silently erode intelligence, disrupt behaviours, truncate future achievements, and damage societies, perhaps most seriously in developing countries. The latter is called the "Silent Pandemic of Neurodevelopmental Toxicity in Children" (7,8). Recently, the aluminium content of brain tissue in autism spectrum disorder was found to be consistently high (9), and the prevalence of autism spectrum disorder is increasing, last CDC estimated prevalence is 1 in 44 children (10).

Worldwide land precipitation of aluminium following aerosol spraying into the lower stratosphere would increase human body exposure and seriously threaten Global Mental Health.

1) Shepherd, J. Geoengineering the Climate: Science, Governance and Uncertainty (The Royal Society, 2009). Albedo Modification Strategies. Climate Intervention by Stratospheric Aerosol Albedo Modification (SAAM), pag. 66. 2) D. K. Weisenstein, D. W. Keith, and J. A. Dykema. Solar geoengineering using solid aerosol in the stratosphere. Atmos. Chem. Phys., 15, 11835–11859, 2015. 3) F. D. Pope, P. Braesicke, R. G. Grainger, M. Kalberer, I. M. Watson, P. J. Davidson and R. A. Cox Stratospheric aerosol particles and solar-radiation management. Nature Climate Change. 2012;2(10):713-719. 4) Trisos HC, Amatulli G, Gurevitch J, Robock A, Xia L and Zambri B. Potentially dangerous consequences for biodiversity of solar geoengineering implementation and termination. Nature Ecology & Evolution volume 2, pages475–482 (2018). 5) World Health Organization. Global action plan on the public health response to dementia. 2017-2025. 6) Bondy SC. Prolonged exposure to low levels of aluminium leads to changes associated with brain ageing and neurodegeneration. Toxicology 315 (2014) 1-7. 7) Grandjean P, Landrigan PJ. Developmental neurotoxicity of industrial chemicals. Lancet. 2006 Dec 16;368(9553):2167-78. 8) Grandjean P, Landrigan PJ. Neurobehavioural effects of developmental toxicity. The Lancet Neurology, Volume 13, Issue 3, Pages 330 - 338, March 2014. 9) Mold M, et al. aluminium in brain tissue in autism. J Trace Elem Med Biol. 2018. 10) CDC. National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention. Autism and Developmental Disabilities Monitoring (ADDM) Network. 31 March 2022. Accessed 8 April 2022.

Competing interests: No competing interests