Effiong and Neitzel Environmental Health (2016) 15:7 
DOI 10.1186/si 2940-016-0089-0 


Environmental Health 


COMMENTARY 


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


Utibe Effiong and Richard L. Neitzel © 


Open Access 




CrossMark 


Abstract 

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 recommendations 
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 

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 be¬ 
come significant geological forces, the term “anthropo- 
cene” has been applied to the current geological epoch, 
which began in the eighteenth century [3]. The United 
Nations Intergovernmental Panel on Climate Change 
(IPCC) has forecast that if human activity and world de¬ 
velopment continue unimpeded, average surface temper¬ 
atures could rise as much as 48 °C by 2100 [1, 2, 4]. 
The lack of success to date in efforts to reduce green¬ 
house gas emissions sufficiently has prompted attention 


* Correspondence: rneitzel@umich.edu 

Department of Environmental Health Sciences, University of Michigan, 1415 
Washington Heights, Ann Arbor, Ml 48109, USA 

Bio IVIed Central 


to the possibility of counteracting the effects of emis¬ 
sions 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 geoengi¬ 
neering as applied in its current context was introduced 
in 1977 [7]. Geoengineering approaches include solar ra¬ 
diation management, or SRM, and carbon dioxide re¬ 
moval (CDR) [5]. SRM techniques attempt to offset 
effects of increased greenhouse gas concentrations by re¬ 
ducing the proportion of incoming short wavelength 
solar radiation that is absorbed or reflected by the earths 
atmosphere (Fig. 1) [8]. Proposed SRM techniques in¬ 
clude stratospheric aerosols, reflective satellites, whiten¬ 
ing of the clouds, whitening of built structures and 


© 2016 Effiong and Neitzel. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 
International License (http://creativecommons.Org/licenses/by/4.0/), which permits unrestricted use, distribution, and 
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(http://creativecommons.org/publicdomain/zero/TO/) applies to the data made available in this article, unless otherwise stated. 






Effiong and Neitzel Environmental Health (2016) 15:7 


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( N 


EARTH S RADIATION BUDGET 


m m 

rnranfWisniirf) Risrrixnssnrfffjfis crr™ rrr3r?rs 



Fig. 1 Components of the earth's radiation budget (adapted from NASA. http://science-edu.larc.nasa.gov/EDDOCS/whatis.html) 

v_____ ) 


increasing plant reflectivity (Fig. 2) [5]. All SRM deploy¬ 
ment techniques require a global approach since local¬ 
ized 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 afforest¬ 
ation/reforestation, direct air carbon dioxide (C0 2 ) cap¬ 
ture/storage, manufacturing carbonate minerals using 
silicate rocks and C0 2 from the air, accelerated weather¬ 
ing of rocks, ocean alkalinity addition and ocean 
fertilization (Fig. 2) [5]. 

This paper will focus on SRM via stratospheric aerosol 
injection, and will describe potential direct human 
health impacts. We explore three knowledge gaps: 1) hu¬ 
man exposures, 2) human health impacts, and 3) expos¬ 
ure limits. SRM may be expected to result in ecosystem 
damage and resulting human health effects through in¬ 
direct 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 Earths 
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 demon¬ 
strated by global cooling following large volcanic 
eruptions [10]. 

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 cur¬ 
rently 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 are considered very promising. 
The particles would utilize photophoretic and electro¬ 
magnetic forces to self-levitate above the stratosphere 
[11]. These nanoparticles would remain suspended lon¬ 
ger 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 in¬ 
crease ocean acidification due to atmospheric C0 2 en¬ 
trapment, has uncharacterized human health and 
environmental impacts, and may be prohibitively expen¬ 
sive [12]. 

Knowledge gap 1: human exposures 

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






Effiong and Neitzel Environmental Health (2016) 15:7 


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SPACE MIRRORS 

Mirrors reflect sunlight from Earth orbit 


wW 

biU^ 


AEROSOLS 



> 


Stratospheric particles reflect sunlight 


REFLECTIVE CROPS 

Plants with reflective 
leaves 


ARTIFICIAL TREES 

"Trees" designed to extract 
and store C0 2 from air 


FORESTING 

Planting of additional trees 
to extract C0 2 from air 


CLOUD SEEDING 

Creation of clouds from seawater 
to reflect sunlight 



BIOCHAR 


OCEAN FERTILISATION 


CARBONATE ADDITION 


Burning and burial of carbonaceous Addition of iron fillings to promote growth 
agricultural waste of C0 2 -extracting plankton 


Addition of ground limestone 
to promote oceanic C0 2 extraction 


Fig. 2 Potential methods for solar radiation management and carbon dioxide removal (adapted 
from http://r3zn8d.files.wordpress.com/2013/04/geoengineering.jpg) 

v V 


Occupational exposures 

Airborne sulfate exposures have been shown to range 
up to 23 mg/m 3 in sulfuric acid plants [14]. Addition¬ 
ally, 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 opera¬ 
tions [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 ex¬ 
posure to barium titanate. In addition to manufactur¬ 
ing settings, exposures to SRM materials could occur 
during deployment, e.g., during cloud seeding opera¬ 
tions, 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 ef¬ 
fects 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 pro¬ 
tective equipment (PPE) such as respirators and protect¬ 
ive suits. 


Population exposures 

Due to atmospheric circulation and gravitational 
deposition, large-scale population exposures to 
atmospherically-injected SRM materials will almost cer¬ 
tainly occur after their deployment. Population expo¬ 
sures could also occur through ingestion of food and 
water contaminated with deposited particles, as well as 
transdermally [11, 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 envir¬ 
onmental 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 es¬ 
timated that with 1 Tg of black carbon infused into 
the stratosphere annually, after ten years of geoengi¬ 
neering, 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 ex¬ 
acerbate adverse health effects already resulting from 
unintentional release at ground level [24]. In the year 
2000, the global emission of black carbon was esti¬ 
mated at 7.6 Tg, and the globally averaged mass bur¬ 
den of black carbon was roughly 1.5 x 10“ 5 kg m -2 
[25]. No models appear to have estimated the 



















Effiong and Neitzel Environmental Health (2016) 15:7 


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potential global burden of environmental aluminum, 
alumina or barium titanate that might result from 
SRM. 

In contrast to occupational exposures, population ex¬ 
posures to SRM materials will be continuous and pro¬ 
longed over months to years, but will likely be orders of 
magnitude lower than those experienced occupationally. 
Thus the health effects will be primarily chronic in na¬ 
ture. The use of PPE to reduce personal exposures to de¬ 
posited 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. 

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 epithe¬ 
lium has been observed in animal studies at exposures 
as low as 0.3 mg/m 3 , with more severe metaplasia fol¬ 
lowing exposures of 1.38 mg/m 3 . Epidemiological studies 
suggest a relationship between exposure to mists con¬ 
taining sulfuric acid and an increased incidence of laryn¬ 
geal cancer, and the International Agency for Research 
on Cancer has concluded that “occupational exposure to 


strong inorganic mists containing sulfuric acid is car¬ 
cinogenic for humans” [27, 28]. 

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, ir¬ 
ritation, increased salivation, and erythema of the trachea 
and main bronchi occurred following controlled expo¬ 
sures 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 obstruc¬ 
tion that may only partially reverse over time [31]. Expo¬ 
sures 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 

[31]. 

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 in¬ 
sensitive 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 


Table 1 Human health effects of the potential SRM aerosols 


Health effect/target 
system 

Potential SRM aerosol 






Sulfuric 

acid 

Sulfur 

dioxide 

Hydrogen 

sulfide 

Carbonyl 

sulfide 

Black 

carbon 

Aluminum 

compounds 

Barium 

compounds 

Respiratory 

X 

X 

X 

X 

X 

X 

X 

Cardiovascular 

X 

X 

X 

X 

X 

X 

- 

G.l 

- 

X 

X 

X 

- 

- 

X 

Hematologic 

- 

X 

X 

X 

X 

X 

- 

Musculosketal 

- 

- 

X 

X 

- 

X 

X 

Hepatic 

- 

- 

- 

- 

X 

- 

- 

Renal 

- 

- 

- 

X 

- 

- 

X 

Endocrine 

- 

- 

- 

- 

- 

X 

- 

Dermal 

X 

X 

X 

X 

- 

- 

- 

Ocular 

X 

X 

X 

X 

X 

- 

- 

Metabolic 

X 

- 

X 

X 

- 

- 

X 

Immunologic 

- 

X 

- 

- 

- 

X 

- 

Neurologic 

X 

X 

X 

X 

X 

X 

X 

Reproductive 

- 

X 

X 

- 

- 

- 

- 

Developmental 

- 

X 

- 

- 

- 

- 

- 

Genotoxic 

- 

X 

- 

- 

- 

- 

- 

Cancer 

X 

- 

- 

- 

X 

X 

- 

Death 


X 

X 

X 

- 

X 

X 


X Data suggest health hazard possible, - insufficient data available 






Effiong and Neitzel Environmental Health (2016) 15:7 


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of appetite, headache, irritability, poor memory and dizzi¬ 
ness may result following exposures >28 mg/m 3 [39], with 
death occuring. > 700 mg/m 3 [40]. 

Limited information is available on the pharmacokin¬ 
etics of carbonyl sulfide, which likely metabolizes to car¬ 
bon 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, head¬ 
ache, vertigo, amnesia, confusion, nausea, vomiting, diar¬ 
rhea, cardiac arrhythmia, weakness, muscle cramps, and 
unconsciousness [43]. Concentrations >1000 ppm can 
cause sudden collapse, convulsions, and death from re¬ 
spiratory paralysis. 

Respiratory effects in black carbon workers include 
cough, sputum production, bronchitis, pneumoconi¬ 
osis, and decrements in lung function, as well as 
tiredness, chest pain, headache, and respiratory irrita¬ 
tion [24, 44, 45]. Black carbon may cause discolor¬ 
ation of eyelids and conjunctivae [46], and is possibly 
carcinogenic to humans (Group 2B); there is inad¬ 
equate evidence of carcinogenicity in humans, but 
sufficient evidence in experimental animals [24]. 

Aluminum is never found free in nature, and instead 
forms metal compounds, complexes, or chelates includ¬ 
ing aluminum oxide [47]. Aluminum and aluminum 
oxide do not appear to differ in toxicity [47]. Wheezing, 
dyspnea, and impaired lung function, as well as pulmon¬ 
ary 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 ex¬ 
posed to aluminum powder, as have decreased red blood 
cell hemoglobin and finger clubbing [50]. Helper T- 
lymphocyte alveolitis and blastic transformation of per¬ 
ipheral blood lymphocytes in the presence of soluble 
aluminum compounds in vitro were found in an individ¬ 
ual exposed to aluminum dust [51]. There is limited evi¬ 
dence of carcinogenicity among workers; the few 
existing studies have been confounded by concurrent ex¬ 
posures 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 as¬ 
sociated 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 uni¬ 
formly assume an 8-h daily exposure period, organiza¬ 
tions 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 ex¬ 
ample, barium titanate) are currently unregulated and/or 
have no recognized occupational exposure assessment 
procedures. All of these issues apply equally to commu¬ 
nity exposure limits. 

The American Conference of Governmental Industrial 
Hygienists (ACGIH) Threshold Limit Values (TLVs) for 
the potential SRM materials shown in Table 2 are con¬ 
sistently lower than those required by the U.S Occupa¬ 
tional Safety and Health Administration (OSHA) or 
recommended by the U.S National Institute for Occupa¬ 
tional 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 pol¬ 
itical feasibility, while the OSHA limits consider tech¬ 
nical and economic feasibility and are subsequently less 
protective [56, 58]. 

For public exposures - which would likely be wide¬ 
spread following SRM efforts - the EPA, European 
Environmental Agency (EE A), 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 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, 61]. 

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 


Effiong and Neitzel Environmental Health (2016) 15:7 


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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) 


Substance 

U.S Occupational Safety and 

Health Administration (mg/m 3 ) a 

U.S National Institute for Occupational 
Safety and Health (mg/m 3 ) a 

American Conference of Governmental 
Industrial Hygienists (mg/m 3 ) a 

Sulfuric acid 

1 

1 


0.2 

Sulfur dioxide 

13 

5.2 


0.7 b 


- 

13.1 b 


- 

Hydrogen Sulfide 

27.9 C 

13.9 C 


1.4 


60.7 d 

- 


\i 

o 

cr 

Carbonyl Sulfide 

- 

- 


12.3 

Black carbon 

3.5 

3.5 


3 

Aluminum aerosol 

15 

10 


1 e 


5 e 

5 e 


- 

Aluminum oxide 

15 

s e 

- 


- 

Barium titanate 

D 

- 


- 


a . Computed from standards specified in parts per million 

b . Short-term exposure limit (15 minutes) 

c . Ceiling limit 

d . 10-minute single period exposure limit 

e . Respirable fraction 


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 administra¬ 
tive controls (e.g., behavior changes). The substantial po¬ 
tential exposures and subsequent health impacts 
associated with SRM efforts based on stratospheric aero¬ 
sols must be considered further before any attempts are 
made at SRM . 


Recommendations 

In order to be effective, SRM efforts involving strato¬ 
spheric 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 frame¬ 
works of economics, risk, politics, and environ¬ 
mental 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 recom¬ 
mend that subsequent cost-benefit analyses for 
geoengineering explicitly consider health impacts 
of SRM [64]. Assessments should further com¬ 
pare the expected health benefits that may result 
from SRM efforts to potential adverse health 


Table 3 Ambient air quality standards for substances that may be utilized in solar radiation management 


Substance 

U.S Environmental Protection Agency 

European Environmental Agency 

World Health Organization 

Limit (pg/m 3 ) 

Averaging period 

Limit (pg/m 3 ) 

Averaging period 

Limit (pg/m 3 ) 

Averaging period 

Sulfuric acid 

- 

- 

- 

- 

- 

- 

Sulfur dioxide 

196.5 

1 h 

350 

1 h 

20 

24 h 




125 

24 h 

500 

10 min 

Hydrogen sulfide 

- 

- 

- 

- 

- 

- 

Carbonyl sulfide 

- 

- 

- 

- 

- 

- 

Nanoparticles 

- 

- 

- 

- 

- 

- 

PM Z5 

12 

1 year 

25 

1 year 

10 

1 year 


35 

24 h 

- 

- 

25 

24 h 











Effiong and Neitzel Environmental Health (2016) 15:7 


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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 tem¬ 
poral movement of SRM materials from the strato¬ 
sphere 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 
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 

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 feas¬ 
ible 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 geoengi¬ 
neering, and there is substantial evidence that a host of 
adverse human health effects will directly result from cli¬ 
mate change, very little has been done to describe the po¬ 
tential human health impacts of this emerging disruptive 
technology. We have described the potential occupational 
and public health impacts of inadvertent exposure to po¬ 
tential 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 stan¬ 
dards that will limit the negative human health impact 
of SRM. Only following a comprehensive risk assess¬ 
ment that addresses each of these issues can the poten¬ 
tial benefits of this geoengineering approach be weighed 
against the potential public health burdens created by 
this technology. 

Abbreviations 

ACGIH: 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; SRM: Solar Radiation Management; TLV: Threshold Limit 
Values. 

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. 

Acknowledgements 

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

Received: 17 July 2015 Accepted: 10 January 2016 
Published online: 19 January 2016 

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