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o 


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ENVIRONMENTAL HEALTH 



Environ Health . 2016; 15: 7. 

Published online 2016 Jan 19. doi: 10.1186/si 2940-016-0089-0 


PMCID: PMC4717532 


Assessing the direct occupational and public health impacts of solar 
radiation management with stratospheric aerosols 


Utibe Effionq and Richard L. NeitzeP 


Department of Environmental Health Sciences, University of Michigan, 1415 Washington Heights, Ann Arbor, Ml 48109 USA 
Utibe Effiong, Email: ueffionq@umich.edu . 

Contributor Information . 


^Corresponding author. 


Received 2015 Jul 17; Accepted 2016 Jan 10. 

Copyright © Effiong and Neitzel. 2016 

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License 
f http://creativecommons.orq/licenses/bv/4.0/ l. which permits unrestricted use, distribution, and reproduction in any medium, provided 
you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if 
changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.orq/publicdomain/zero/1.0/ t 
applies to the data made available in this article, unless otherwise stated. 


Abstract 


Go to: 


Geoengineering is the deliberate large-scale manipulation of environmental processes that affects the 
Earth’s climate, in an attempt to counteract the effects of climate change. Injecting sulfate aerosol 
precursors and designed nanoparticles into the stratosphere to (i.e., solar radiation management 
[SRM]), has been suggested as one approach to geoengineering. Although much is being done to 
unravel the scientific and technical challenges around geoengineering, there have been few efforts to 
characterize the potential human health impacts of geoengineering, particularly with regards to SRM 
approaches involving stratospheric aerosols. This paper explores this information gap. Using available 
evidence, we describe the potential direct occupational and public health impacts of exposures to 
aerosols likely to be used for SRM, including environmental sulfates, black carbon, metallic aluminum, 
and aluminum oxide aerosols. We speculate on possible health impacts of exposure to one promising 
SRM material, barium titanate, using knowledge of similar nanomaterials. We also explore current 
regulatory efforts to minimize exposure to these toxicants. Our analysis suggests that adverse public 
health impacts may reasonably be expected from SRM via deployment of stratospheric aerosols. Little 
is known about the toxicity of some likely candidate aerosols, and there is no consensus regarding 
acceptable levels for public exposure to these materials. There is also little infrastructure in place to 
evaluate potential public health impacts in the event that stratospheric aerosols are deployed for solar 
radiation management. We offer several reco mm endations intended to help characterize the potential 
occupation and public health impacts of SRM, and suggest that a comprehensive risk assessment effort 
is needed before this approach to geoengineering receives further consideration. 

Keywords: Climate change, Geoengineering, Solar radiation management, Aerosols, Exposure, 

Human health 

Background _ Goto: 


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Warming of the climate system is unequivocal, and since the 1950s, human influence on the climate 
system has become clear [1, 2], Because human activities have become significant geological forces, 
the term “anthropocene” has been applied to the current geological epoch, which began in the 
eighteenth century [3], The United Nation’s Intergovernmental Panel on Climate Change (IPCC) has 
forecast that if human activity and world development continue unimpeded, average surface 
temperatures could rise as much as 4.8 °C by 2100 [1, 2, 4], The lack of success to date in efforts to 
reduce greenhouse gas emissions sufficiently has prompted attention to the possibility of counteracting 
the effects of emissions through the intentional manipulation of global-scale Earth system processes - a 
process referred to as “geoengineering” [5] 

The concept of geoengineering is not new, and dates back to at least 1965 [6], However, the term 
geoengineering as applied in its current context was introduced in 1977 [7], Geoengineering 
approaches include solar radiation management, or SRM, and carbon dioxide removal (CDR) [5], SRM 
techniques attempt to offset effects of increased greenhouse gas concentrations by reducing the 
proportion of incoming short wavelength solar radiation that is absorbed or reflected by the earth’s 
atmosphere (Fig. I) [8], Proposed SRM techniques include stratospheric aerosols, reflective satellites, 
whitening of the clouds, whitening of built structures and increasing plant reflectivity (Fig. 2) [5], All 
SRM deployment techniques require a global approach since localized deployment will not produce 
sufficient effects. Importantly, SRM approaches to managing climate change require initial and 
ongoing addition of aerosols to the atmosphere, with increasingly greater additions as emissions of 
GHGs rise, given the risk of sudden and potentially catastrophic warming if aerosol levels are not 
maintained. Proposed CDR approaches include afforestation/reforestation, direct air carbon dioxide 
(CO 2 ) capture/storage, manufacturing carbonate minerals using silicate rocks and CO 2 from the air, 
accelerated weathering of rocks, ocean alkalinity addition and ocean fertilization (Fig. 2) [5]. 


Fig. 1 

Components of the earth’s radiation budget (adapted from NASA. 
http://science-edu.larc.nasa.gov/EDDOCS/whatis.html') 


Fig. 2 

Potential methods for solar radiation management and carbon 
dioxide removal (adapted from 

http://r3zn8d.files.wordpress.com/2013/04/geoengineering.jp gl 

This paper will focus on SRM via stratospheric aerosol injection, and will describe potential direct 
human health impacts. We explore three knowledge gaps: 1) human exposures, 2) human health 
impacts, and 3) exposure limits. SRM may be expected to result in ecosystem damage and resulting 
human health effects through indirect mechanisms such as damage to, or contamination of, agricultural 
products and wildlife. While these effects are important, they are beyond the scope of our paper. 

Stratospheric aerosols for use in SRM 

The stratosphere is the second major layer of Earth’s atmosphere, lying immediately above the lowest 
layer (the troposphere) at an altitude of 10-50 km [9]. Within the stratosphere temperatures increase 
with increasing elevation. The potential for SRM from stratospheric injection of aerosols has been 
demonstrated by global cooling following large volcanic eruptions [101- 

A wide range of particles could be released into the stratosphere to achieve the SRM objective of 
scattering sunlight back to space. Sulfates and nanoparticles currently favored for SRM include sulfur 
dioxide, hydrogen sulfide, carbonyl sulfide, black carbon, and specially engineered discs composed of 
metallic aluminum, aluminum oxide and barium titanate [11]. In particular, engineered nanoparticles 



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are considered very promising. The particles would utilize photophoretic and electromagnetic forces to 
self-levitate above the stratosphere [IT]. These nanoparticles would remain suspended longer than 
sulfate particles, would not interfere with stratospheric chemistry, and would not produce acid rain 
[12]. However, while promising, the self-levitating nanodisc has not been tested to verify efficacy, may 
increase ocean acidification due to atmospheric CO 2 entrapment, has uncharacterized human health 
and environmental impacts, and may be prohibitively expensive [12], 

Knowledge gap 1: human exposures 

Human exposures to materials used for SRM could occur during the manufacture, transportation, 
deployment and post-deployment of these materials [13]. In this paper, unless otherwise stated, 
inhalation is the primary route of exposure considered. 

Occupational exposures 

Airborne sulfate exposures have been shown to range up to 23 mg/m 3 in sulfuric acid plants [14] , 
Additionally, high exposures to sulfuric acid fumes have also been noted in the petrochemical industry, 
and high exposures to hydrogen sulfide and carbonyl sulfide have also been noted in natural gas 
extraction operations [15,16], Exposures to black carbon during its manufacture can be quite high [17] , 
Elevated airborne exposures to aluminum and its oxide have been shown to occur during aluminum 
refining, smelting and at aluminum powder plants [18]. There appears to be no available 
documentation of occupational exposure to barium titanate. In addition to manufacturing settings, 
exposures to SRM materials could occur during deployment, e.g., during cloud seeding operations, as 
well as from accidents during transportation [19, 20 ], 

Occupational exposures to SRM materials are likely to occur over brief periods (e.g., days to weeks), 
with the potential for repeated or cyclic exposures. The health effects of such exposures will therefore 
likely be acute in nature, though repeated exposures create an opportunity for chronic health effects. 
Occupational exposures may be attenuated through the use of engineering controls such as ventilation, 
as well as the use of personal protective equipment (PPE) such as respirators and protective suits. 

Population exposures 

Due to atmospheric circulation and gravitational deposition, large-scale population exposures to 
atmospherically-injected SRM materials will almost certainly occur after their deployment. Population 
exposures could also occur through ingestion of food and water contaminated with deposited particles, 
as well as transdermally [JT, 21]. Unlike occupational exposures, there has been virtually no research 
done to estimate ground-level personal exposures to SRM materials, though the US Environmental 
Protection Agency (EPA) does provide guidance on methods for evaluating environmental exposures to 
several possible SRM materials [22], 

Stratospheric injection of sulfur dioxide and black carbon has already been modeled to analyze 
potential deposition of sulfate and soot [21, 23], One model estimated that with 1 Tg of black carbon 
infused into the stratosphere annually, after ten years of geoengineering, the globally averaged mass 
burden would be approximately 8 x 10 -6 kg m -2 [23]. The intentional addition of black carbon to the 
atmosphere will exacerbate adverse health effects already resulting from unintentional release at 
ground level [24]- In the year 2000, the global emission of black carbon was estimated at 7.6 Tg, and 
the globally averaged mass burden of black carbon was roughly 1.5x10 5 kgm 2 [25] . No models 
appear to have estimated the potential global burden of environmental aluminum, alumina or barium 
titanate that might result from SRM. 

In contrast to occupational exposures, population exposures to SRM materials will be continuous and 
prolonged over months to years, but will likely be orders of magnitude lower than those experienced 


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occupationally. Thus the health effects will be primarily chronic in nature. The use of PPE to reduce 
personal exposures to deposited SRM materials is not feasible on a population scale. 

Knowledge gap 2: potential human health impacts 

Table 1 summarizes, by bodily system, the potential human health effects of the aerosols that may be 
used for SRM. 


Table 1 

Human health effects of the potential SRM aerosols 


Inhalational studies with sulfuric acid aerosol suggest that it has a local irritant effect and no systemic 
effects [26], Squamous cell metaplasia in the laryngeal epithelium has been observed in animal studies 
at exposures as low as 0.3 mg/m 3 , with more severe metaplasia following exposures of 1.38 mg/m 3 . 
Epidemiological studies suggest a relationship between exposure to mists containing sulfuric acid and 
an increased incidence of laryngeal cancer, and the International Agency for Research on Cancer has 
concluded that “occupational exposure to strong inorganic mists containing sulfuric acid is 
carcinogenic for humans” [27, 281 . 

In humans, and in particular asthmatics, increases in specific airway resistance or decreases in forced 
expiratory volume or forced expiratory flow are the primary response following acute exposure to 
sulfur dioxide [29], Cough, irritation, increased salivation, and erythema of the trachea and main 
bronchi occurred following controlled exposures to <8 ppm for 20 min [30], Exposures to higher levels 
(e.g., 40 ppm) can produce a burning sensation in the nose and throat, dyspnea, and severe airway 
obstruction that may only partially reverse over time [31]. Exposures to even higher levels (e.g., 

<100 ppm) can result in reactive airway dysfunction syndrome, which involves bronchial epithelial 
damage and increased sensitization and nonspecific hypersensitivity to other irritant stimuli [32, 33 ], 
Deaths can occur following exposures >100 ppm [311 . 

Single exposures to hydrogen sulfide can cause health effects in many systems [34]- Hydrogen sulfide 
has an odor threshold of 0.01 mg/m 3 , and humans become insensitive to its odor at concentrations of 
>140 mg/m 3 [35, 36]- Respiratory symptoms in asthmatic individuals appear at about 2.8 mg/m 3 , but 
respiratory distress does not seem to occur <560 mg/m 3 [37]. Eye irritation can occur at 5-29 mg/m 3 , 
and metabolic abnormalities may occur at 7 mg/m 3 [38], Neurological symptoms such as fatigue, loss 
of appetite, headache, irritability, poor memory and dizziness may result following exposures 
>28 mg/m 3 [39], with death occuring. > 700 mg/m 3 [40], 

Limited information is available on the pharmacokinetics of carbonyl sulfide, which likely metabolizes 
to carbon dioxide and hydrogen sulfide [41] . Acute exposures result in symptoms similar to those of 
hydrogen sulfide, but with less local irritation or olfactory warning [42]. Sublethal exposure can result 
in profuse salivation, headache, vertigo, amnesia, confusion, nausea, vomiting, diarrhea, cardiac 
arrhythmia, weakness, muscle cramps, and unconsciousness [43.]. Concentrations >1000 ppm can cause 
sudden collapse, convulsions, and death from respiratory paralysis. 

Respiratory effects in black carbon workers include cough, sputum production, bronchitis, 
pneumoconiosis, and decrements in lung function, as well as tiredness, chest pain, headache, and 
respiratory irritation [24, 44, 45], Black carbon may cause discoloration of eyelids and conjunctivae 
[46] , and is possibly carcinogenic to humans (Group 2B); there is inadequate evidence of 
carcinogenicity in humans, but sufficient evidence in experimental animals [24], 


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Aluminum is never found free in nature, and instead forms metal compounds, complexes, or chelates 
including aluminum oxide [47], Aluminum and aluminum oxide do not appear to differ in toxicity [47], 
Wheezing, dyspnea, and impaired lung function, as well as pulmonary fibrosis, have been noted in 
workers exposed to fine aluminum dust [48-50]- Dilation and hypertrophy of the right side of the heart 
have been seen in workers exposed to aluminum powder, as have decreased red blood cell hemoglobin 
and finger clubbing [50] . Helper T-lymphocyte alveolitis and blastic transformation of peripheral blood 
lymphocytes in the presence of soluble aluminum compounds in vitro were found in an individual 
exposed to aluminum dust [51] . There is limited evidence of carcinogenicity among workers; the few 
existing studies have been confounded by concurrent exposures to known carcinogens, (e.g., tobacco 
smoke or polycyclic aromatic hydrocarbons) [52], 

Barium titanate is a complex salt containing two metals, which complicates modeling of its 
toxicological properties. In general, exposures to barium salts are associated with respiratory, 
cardiovascular, gastrointestinal, musculoskeletal, metabolic and neurologic effects [53] , Barium salts 
also have a local effect on skin surfaces and would not likely be absorbed systematically to any great 
extent, though this might not be true of barium salt nanoparticles [53, 54], Barium titanate could also 
behave like a titanium salt in interactions with the human body, in which case the resulting health 
effects are essentially unknown. Only two titanium-containing compounds are indexed by the U.S 
Agency for Toxic Substances and Disease Registry (ATSDR) or covered by U.S exposure limits [55]. It 
is possible that barium titanate might act both as a salt of barium and titanium, or as neither; the 
toxicological properties of a nanoparticle are influenced by factors such as particle size, surface area, 
chemistry or reactivity, solubility, and shape [54], 


Knowledge gap 3: exposure standards and guidelines 

Several US agencies and organizations have established occupational exposure limits (OELs) for 
sulfate, carbon, and some metallic substances. While OELs almost uniformly assume an 8-h daily 
exposure period, organizations use different assumptions and acceptable excess risk levels when 
establishing limits. As a result there are a range of OELs for potential SRM materials, which 
complicates the establishment of “safe” global levels. Additionally, some potential SRM compounds 
(for example, barium titanate) are currently unregulated and/or have no recognized occupational 
exposure assessment procedures. All of these issues apply equally to community exposure limits. 

The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values 
(TLVs) for the potential SRM materials shown in Table 2 are consistently lower than those required by 
the U.S Occupational Safety and Health Administration (OSHA) or recommended by the U.S National 
Institute for Occupational Safety and Health (NIOSH) [56, 57] The TLVs and NIOSH Recommended 
Exposure Limits (RELs) are intended to protect the typical worker from any adverse health effects 
without consideration of economic or political feasibility, while the OSHA limits consider technical 
and economic feasibility and are subsequently less protective [56, 58] , 



Table 2 

Occupational exposure standards for substances that may be 
utilized in solar radiation management (Unless otherwise 
specified, exposure limits are average levels over an 8-h workday) 


For public exposures - which would likely be widespread following SRM efforts - the EPA, European 
Environmental Agency (EEA), and World Health Organization specify regulatory standards for 
ambient air quality (Table 3) [57-59], Importantly, Table 3 shows a very small sampling of air quality 
standards in use around the world that relate to potential SRM materials, of which the WHO standards 
may be considered most generalizable globally. Exposure limits differ substantially between these 


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agencies, but, more importantly, there are currently no limits set by any of these agencies for most of 
the substances that may be used for SRM [60, 611 - 



Table 3 

Ambient air quality standards for substances that may be utilized 
in solar radiation management 


The inconsistencies in established exposure limits for both occupational and community settings, 
combined with the absence of any exposure limits for a number of potential SRM materials, highlight 
the issues involved in protecting workers and the public from unintended health consequences resulting 
from SRM deployment. Since employers have legal control over exposures to their workers, OELs can 
be met through implementation of engineering controls and use of PPE, whereas use of PPE is not 
feasible at a population level, and reductions in public exposures would have to rely on engineering 
controls (e.g., use of air cleaning devices) or administrative controls (e.g., behavior changes). The 
substantial potential exposures and subsequent health impacts associated with SRM efforts based on 
stratospheric aerosols must be considered further before any attempts are made at SRM . 

Recommendations 

In order to be effective, SRM efforts involving stratospheric aerosols will require a global effort. Such 
an action would represent the first truly global and intentionally-produced human exposures, and 
because the benefits and potential consequences of this action would impact the entire population of the 
planet to some degree, we make the following initial recommendations: 

i. Geoengineering cost-benefit analyses should consider health impacts of SRM. 

At present, most assessments of geoengineering are done within specific and well-defined 
frameworks of economics, risk, politics, and environmental ethics [62], Literature on the 
potential human health impacts of SRM is scant, and such impacts have not been adequately 
factored into previous cost-benefit analyses [63], We recommend that subsequent cost-benefit 
analyses for geoengineering explicitly consider health impacts of SRM [64], Assessments should 
further compare the expected health benefits that may result from SRM efforts to potential 
adverse health outcomes, including (but not limited to) those described here. 

ii. Further research is needed on methods of assessment of exposures to, and evaluation of 
toxicological properties of, potential SRM materials. 

We have noted gaps in current scientific knowledge related to occupational and community 
exposures that would result from SRM, as well to the toxicological properties of potential SRM 
materials. Additional laboratory- and field-based research is needed in these areas, particularly 
with regard to exposure characterization and the spatial and temporal movement of SRM 
materials from the stratosphere to ground level. While it is difficult to develop exposure and 
toxicological models which are representative of a decades- or centuries-long SRM deployment, 
these efforts are critical to ensure that reasonable, validated models of exposures and human 
health impacts are available prior to any SRM deployment. 

iii. Strict and harmonized global occupational and community exposure limits are needed for SRM 
materials. 

Tables 2 and 3 illustrate the divergence and incompleteness of current occupational and 
community exposure limits regarding potential SRM materials. Since exposures will inherently 
be global in nature, exposure limits must be harmonized to ensure that individuals around the 
world are given equal protection from adverse health effects. Global harmonization of standards 
related to SRM represents an immense but necessary bureaucratic and scientific challenge, and 
an important step towards establishing a formal governance framework for geoengineering. A 


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global discussion of standards harmonization relating to SRM may result in other tangible 
benefits to society, including the potential evolution of a common language and framework for 
risk assessment and a debate on the strengths and weaknesses of different approaches to risk 
management. 

iv. Reversal mechanisms should be identified prior to any SRM deployment 

In the event that substantial health impacts are noted following deployment of stratospheric 
aerosol approaches to SRM, mechanisms for capturing the aerosols to halt further ground-level 
exposures through gravitational deposition will be needed. Therefore, if stratospheric aerosols 
are pursued as a viable SRM strategy, such mechanisms will need to be identified and evaluated 
prior to large-scale deployment. 

Conclusion Goto: 

Although there is very little agreement in the scientific community on the approach to SRM-related 
technologies, SRM has been identified as a potentially technically feasible and possibly cost-effective 
method of geoengineering to reduce or reverse anthropogenically-driven climate change [1, 62], But 
even as much is being done to unravel the scientific and technical challenges around geoengineering, 
and there is substantial evidence that a host of adverse human health effects will directly result from 
climate change, very little has been done to describe the potential human health impacts of this 
emerging disruptive technology. We have described the potential occupational and public health 
impacts of inadvertent exposure to potential SRM materials, and have also speculated on the possible 
health impacts of exposure to barium titanate using knowledge of similar nanomaterials. 

Based on our analyses, we submit that the current knowledge gaps do not justify deployment of SRM 
in the short term. We therefore recommend further research, a more inclusive analysis of costs and 
benefits, as well as the globalization and harmonization of regulatory standards that will limit the 
negative human health impact of SRM. Only following a comprehensive risk assessment that addresses 
each of these issues can the potential benefits of this geoengineering approach be weighed against the 
potential public health burdens created by this technology. 

Acknowledgements _ Goto: 

Funding for this study was provided by the University of Michigan MCubed funding program and by 
the University of Michigan Risk Science Center. 

Abbreviations Goto: 


ACGIFI American Conference of Governmental Industrial Hygienist 

ATSDR U.S Agency for Toxic Substances and Disease Registry 

CDR Carbon Dioxide Removal 

EPA U.S Environmental Protection Agency 

IPCC Intergovernmental Panel on Climate Change 

NIOSH National Institute for Occupational Safety and Health 

OSHA Occupational Safety and Health Administration 

PPE Personal Protective Equipment 

REL Recommended Exposure Limits 


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SRM Solar Radiation Management 

TLV Threshold Limit Values 

Footnotes Goto: 

Competing interests 

The authors have no competing financial interests to declare. 

Authors’ contributions 

UE carried out the literature review and drafted the manuscript. RN conceived of the study, participated in its design 
and coordination, and helped draft the manuscript. Both authors read and approved the final manuscript. 

Contributor Information Goto: 

Utibe Effiong, Email: ueffiong@umich.edu . 

Richard L. Neitzel, Phone: (734) 763-2870, Email: rneitzel@umich.edu . 

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