Imperial College London

Sierra Clark

Faculty of MedicineSchool of Public Health

Honorary Research Associate







171Norfolk PlaceSt Mary's Campus





Publication Type

10 results found

Clark S, Alli AS, Ezzati M, Brauer M, Toledano M, Nimo J, Bedford Moses J, Baah S, Hughes A, Cavanaugh A, Agyei-Mensah S, Owusu G, Robinson B, Baumgartner J, Bennett J, Arku Ret al., 2022, Spatial modelling and inequalities of environmental noise in Accra, Ghana, Environmental Research, Vol: 214, ISSN: 0013-9351

Noise pollution is a growing environmental health concern in rapidly urbanizing sub-Saharan African (SSA) cities. However, limited city-wide data constitutes a major barrier to investigating health impacts as well as implementing environmental policy in this growing population. As such, in this first of its kind study in West Africa, we measured, modelled and predicted environmental noise across the Greater Accra Metropolitan Area (GAMA) in Ghana, and evaluated inequalities in exposures by socioeconomic factors. Specifically, we measured environmental noise at 146 locations with weekly (n = 136 locations) and yearlong monitoring (n = 10 locations). We combined these data with geospatial and meteorological predictor variables to develop high-resolution land use regression (LUR) models to predict annual average noise levels (LAeq24hr, Lden, Lday, Lnight). The final LUR models were selected with a forward stepwise procedure and performance was evaluated with cross-validation. We spatially joined model predictions with national census data to estimate population levels of, and potential socioeconomic inequalities in, noise levels at the census enumeration-area level. Variables representing road-traffic and vegetation explained the most variation in noise levels at each site. Predicted day-evening-night (Lden) noise levels were highest in the city-center (Accra Metropolis) (median: 64.0 dBA) and near major roads (median: 68.5 dBA). In the Accra Metropolis, almost the entire population lived in areas where predicted Lden and night-time noise (Lnight) surpassed World Health Organization guidelines for road-traffic noise (Lden <53; and Lnight <45). The poorest areas in Accra also had significantly higher median Lden and Lnight compared with the wealthiest ones, with a difference of ∼5 dBA. The models can support environmental epidemiological studies, burden of disease assessments, and policies and interventions that address underlying causes of noise exposure ineq

Journal article

Suel E, Sorek-Hamer M, Moise I, Von Pohle M, Sahasrabhojanee A, Asanjan AA, Arku RE, Alli AS, Barratt B, Clark SN, Middel A, Deardorff E, Lingenfelter V, Oza NC, Yadav N, Ezzati M, Brauer Met al., 2022, What you see is what you breathe? Estimating air pollution spatial variation using street level imagery, Remote Sensing, Vol: 14, ISSN: 2072-4292

High spatial resolution information on urban air pollution levels is unavailable in many areas globally, partially due to high input data needs of existing estimation approaches. Here we introduce a computer vision method to estimate annual means for air pollution levels from street level images. We used annual mean estimates of NO2 and PM2.5 concentrations from locally calibrated models as labels from London, New York, and Vancouver to allow for compilation of a sufficiently large dataset (~250k images for each city). Our experimental setup is designed to quantify intra and intercity transferability of image-based model estimates. Performances were high and comparable to traditional land-use regression (LUR) and dispersion models when training and testing on images from the same city (R2 values between 0.51 and 0.95 when validated on data from ground monitoring stations). Like LUR models, transferability of models between cities in different geographies is more difficult. Specifically, transferability between the three cities i.e., London, New York, and Vancouver, which have similar pollution source profiles were moderately successful (R2 values between zero and 0.67). Comparatively, performances when transferring models trained on these cities with very different source profiles i.e., Accra in Ghana and Hong Kong were lower (R2 between zero and 0.21) suggesting the need for local calibration with local calibration using additional measurement data from cities that share similar source profiles.

Journal article

Clark SN, Bennett JE, Arku RE, Hill AG, Fink G, Adanu RM, Biritwum RB, Darko R, Bawah A, Duda RB, Ezzati Met al., 2021, Small area variations and factors associated with blood pressure and body-mass index in adult women in Accra, Ghana: Bayesian spatial analysis of a representative population survey and census data, PLOS MEDICINE, Vol: 18, ISSN: 1549-1277

Journal article

Wang J, Alli AS, Clark S, Hughes A, Ezzati M, Beddows A, Vallarino J, Nimo J, Bedford-Moses J, Baah S, Owusu G, Agyemang E, Kelly F, Barratt B, Beevers S, Agyei-Mensah S, Baumgartner J, Brauer M, Arku REet al., 2021, Nitrogen oxides (NO and NO2) pollution in the Accra metropolis: Spatiotemporal patterns and the role of meteorology, SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 803, ISSN: 0048-9697

Journal article

Alli AS, Clark S, Hughes AF, Nimo J, Bedford-Moses J, Baah S, Wang J, Vallarino J, Agyemang E, Barratt B, Beddows A, Kelly F, Owusu G, Baumgartner J, Brauer M, Ezzati M, Agyei-Mensah S, Arku REet al., 2021, Spatial-temporal patterns of ambient fine particulate matter (PM2.5) and black carbon (BC) pollution in Accra, Environmental Research Letters, Vol: 16, Pages: 1-12, ISSN: 1748-9326

Background: Sub-Saharan Africa (SSA) is rapidly urbanizing, and ambient air pollution has emerged as a major environmental health concern in SSA cities. Yet, effective air quality management is hindered by limited data. We deployed robust, low-cost and low-power devices in a large-scale measurement campaign and characterized within-city variations in fine particulate matter (PM2.5) and black carbon (BC) pollution in Accra, Ghana. Methods: Between April 2019 and June 2020, we measured weekly gravimetric (filter-based) and minute-by-minute PM2.5 concentrations at 146 unique locations, comprising of 10 fixed (~1-year) and 136 rotating (7-day) sites covering a range of land-use and source influences. Filters were weighed for mass, and light absorbance (10−5m−1) of the filters was used as proxy for BC concentration. Year-long data at four fixed sites that were monitored in a previous study (2006-2007) were compared to assess change in PM2.5 concentrations. Results: The mean annual PM2.5 across the fixed sites ranged from 26 μg/m3 at a peri-urban site to 40 μg/m3 at commercial, business, and industrial (CBI) areas. CBI areas had the highest PM2.5 levels (mean: 37 μg/m3), followed by high-density residential neighborhoods (mean: 36 μg/m3), while peri-urban areas recorded the lowest (mean: 26 μg/m3). Both PM2.5 and BC levels were highest during the dry dusty Harmattan period (mean PM2.5: 89 μg/m3) compared to non-Harmattan season (mean PM2.5: 23 μg/m3). PM2.5 at all sites peaked at dawn and dusk, coinciding with morning and evening heavy traffic. We found about a ~50% reduction (71 vs 37 μg/m3) in mean annual PM2.5 concentrations when compared to measurements in 2006-2007 in Accra. Conclusion: Ambient PM2.5 concentrations in Accra may have plateaued at levels lower than those seen in large Asian megacities. However, levels are still 2- to 4-fold higher than the WHO guideline. Effective and equitable policies are needed to reduce pollution

Journal article

Clark S, Alli A, Nathvani R, Hughes A, Ezzati M, Brauer M, Toledano M, Baumgartner J, Bennett J, Nimo J, Bedford Moses J, Baah S, Agyei-Mensah S, Owusu G, Croft B, Arku Ret al., 2021, Space-time characterization of community noise and sound sources in Accra, Ghana, Scientific Reports, Vol: 11, Pages: 1-14, ISSN: 2045-2322

Urban noise pollution is an emerging public health concern in growing cities in sub-Saharan Africa (SSA), but the sound environment in SSA cities is understudied. We leveraged a large-scale measurement campaign to characterize the spatial and temporal patterns of measured sound levels and sound sources in Accra, Ghana. We measured sound levels and recorded audio clips at 146 representative locations, involving 7-days (136 locations) and 1-year measurements between 2019 and 2020. We calculated metrics of noise levels and intermittency and analyzed audio recordings using a pre-trained neural network to identify sources. Commercial, business, and industrial areas and areas near major roads had the highest median daily sound levels (LAeq24hr: 69 dBA and 72 dBA) and the lowest percentage of intermittent sound; the vice-versa was found for peri urban areas. Road-transport sounds dominated the overall sound environment but mixtures of other sound sources, including animals, human speech, and outdoor music, dominated in various locations and at different times. Environmental noise levels in Accra exceeded both international and national health-based guidelines. Detailed information on the acoustical environmental quality (including sound levels and types) in Accra may guide environmental policy formulation and evaluation to improve the health of urban residents.

Journal article

Loo RL, Lu Q, Carter E, Liu S, Clark S, Wang Y, Jill B, Huiru T, Chan Qet al., 2021, A feasibility study of metabolic phenotyping of dried blood spot specimens in rural Chinese women exposed to household air pollution, Journal of Exposure Science and Environmental Epidemiology, Vol: 31, Pages: 328-344, ISSN: 1559-0631

Background: Exposure-response studies and policy evaluations of household air pollution (HAP) are limited by current methods of exposure assessment which are expensive and burdensome to participants.Methods: We collected 152 dried blood spot (DBS) specimens during the heating and non-heating seasons from 53 women who regularly used biomass-burning stoves for cooking and heating. Participants were enrolled in a longitudinal study in China. Untargeted metabolic phenotyping of DBS were generated using ultra-high performance liquid chromatography coupled with mass spectrometry to exemplify measurement precision and assessment for feasibility to detect exposure to HAP, evaluated by season (high pollution versus low pollution) and measured personal exposure to fine particulate matter <2.5µm diameters (PM) and black carbon (BC) in the 48-h prior to collecting the DBS specimen.Results: Metabolites e.g., amino acids, acyl-carnitines, lyso-phosphorylcholines, sphinganine, and choline were detected in the DBS specimens. Our approach is capable of detecting the differences in personal exposure to HAP whilst showing high analytical reproducibility, coefficient of variance (CV) <15%, meeting the U.S. Food and Drug Administration criteria.Conclusions: Our results provide a proof of principle that high resolution metabolic phenotypic data can be generated using a simple DBS extraction method thus remote, low-resource settings where the collection of serum and plasma is logistically challengingor infeasible. The analytical run time (19 mins/specimen) is similar to most global phenotyping methods and therefore suitable for large-scale application.

Journal article

Clark S, Alli AS, Brauer M, Ezzati M, Baumgartner J, Toledano M, Hughes A, Nimo J, Moses J, Terkpertey S, Vallarino J, Agyei-Mensah S, Agyemang E, Nathvani R, Muller E, Bennett J, Wang J, Beddows A, Kelly F, Barratt B, Beevers S, Arku Ret al., 2020, High-resolution spatiotemporal measurement of air and environmental noise pollution in sub-Saharan African cities: Pathways to Equitable Health Cities Study protocol for Accra, Ghana, BMJ Open, Vol: 10, ISSN: 2044-6055

Introduction: Air and noise pollution are emerging environmental health hazards in African cities, with potentially complex spatial and temporal patterns. Limited local data is a barrier to the formulation and evaluation of policies to reduce air and noise pollution. Methods and analysis: We designed a year-long measurement campaign to characterize air and noise pollution and their sources at high-resolution within the Greater Accra Metropolitan Area, Ghana. Our design utilizes a combination of fixed (year-long, n = 10) and rotating (week-long, n = ~130) sites, selected to represent a range of land uses and source influences (e.g. background, road-traffic, commercial, industrial, and residential areas, and various neighbourhood socioeconomic classes). We will collect data on fine particulate matter (PM2.5), nitrogen oxides (NOx), weather variables, sound (noise level and audio) along with street-level time-lapse images. We deploy low-cost, low-power, lightweight monitoring devices that are robust, socially unobtrusive, and able to function in the Sub-Saharan African (SSA) climate. We will use state-of-the-art methods, including spatial statistics, deep/machine learning, and processed-based emissions modelling, to capture highly resolved temporal and spatial variations in pollution levels across Accra and to identify their potential sources. This protocol can serve as a prototype for other SSA cities. Ethics and dissemination: This environmental study was deemed exempt from full ethics review at Imperial College London and the University of Massachusetts Amherst; it was approved by the University of Ghana Ethics Committee. This protocol is designed to be implementable in SSA cities to map environmental pollution to inform urban planning decisions to reduce health harming exposures to air and noise pollution. It will be disseminated through local stakeholder engagement (public and private sectors), peer-reviewed publications, contribution to policy documents, media, a

Journal article

Clark SN, Schmidt AM, Carter EM, Schauer JJ, Yang X, Ezzati M, Daskalopoulou SS, Baumgartner Jet al., 2019, Longitudinal evaluation of a household energy package on blood pressure, central hemodynamics, and arterial stiffness in China, Environmental Research, Vol: 177, Pages: 1-11, ISSN: 0013-9351

BackgroundCardiovascular diseases are the leading contributors to disease burden in China and globally, and household air pollution exposure is associated with risk of cardiovascular disease.ObjectivesWe evaluated whether subclinical cardiovascular outcomes in adult Chinese women would improve after distribution of an energy package comprised of a semi-gasifier cookstove, water heater, chimney, and supply of processed biomass fuel.MethodsWe enrolled 204 households (n = 205 women) from 12 villages into a controlled before- and after-intervention study on cardiovascular health and air pollution in Sichuan Province. The intervention was distributed to 124 households during a government-sponsored rural energy demonstration program. The remaining 80 households received the package 18 months later at the end of the study, forming a comparison group. One woman from each household had their blood pressure (BP), central hemodynamics, and arterial stiffness measured along with exposures to air pollution and demographic and household characteristics, on up to five visits. We used a difference-in-differences mixed-effects regression approach with Bayesian inference to assess the impact of the energy package on sub-clinical cardiovascular outcomes.ResultsWomen who did not receive the energy package had greater mean decreases in brachial systolic (−4.1 mmHg, 95% credible interval (95%CIe) −7.3, −0.9) and diastolic BP (−2.0 mmHg, 95%CIe −3.6, −0.5) compared with women who received the package (systolic: −2.7, 95%CIe −5.0, −0.4; diastolic: −0.3, 95%CIe −1.4, 0.8) resulting in slightly positive but not statistically significant difference-in-differences effect estimates of 1.3 mmHg (95%CIe −2.5, 5.2) and 1.7 mmHg (95%CIe −0.3, 3.6), respectively. Similar trends were found for central BP, central pulse pressure, and arterial stiffness. Air pollution exposures decreased on average for both treatment groups

Journal article

Clark S, Carter E, Shan M, Ni K, Niu H, Tseng JTW, Pattanayak SK, Jeuland M, Schauer JJ, Ezzati M, Wiedinmyer C, Yang X, Baumgartner Jet al., 2017, Adoption and use of a semi-gasifier cooking and water heating stove and fuel intervention in the Tibetan Plateau, China, Environmental Research Letters, Vol: 12, ISSN: 1748-9326

Improved cookstoves and fuels, such as advanced gasifier stoves, carry the promise of improving health outcomes, preserving local environments, and reducing climate-forcing air pollutants. However, low adoption and use of these stoves in many settings has limited their benefits. We aimed to improve the understanding of improved stove use by describing the patterns and predictors of adoption of a semi-gasifier stove and processed biomass fuel intervention in southwestern China. Of 113 intervention homes interviewed, 79% of homes tried the stove, and the majority of these (92%) continued using it 5–10 months later. One to five months after intervention, the average proportion of days that the semi-gasifier stove was in use was modest (40.4% [95% CI 34.3–46.6]), and further declined over 13 months. Homes that received the stove in the first batch used it more frequently (67.2% [95% CI 42.1−92.3] days in use) than homes that received it in the second batch (29.3% [95% CI 13.8−44.5] days in use), likely because of stove quality and user training. Household stove use was positively associated with reported cooking needs and negatively associated with age of the main cook, household socioeconomic status, and the availability of substitute cleaner-burning stoves. Our results show that even a carefully engineered, multi-purpose semi-gasifier stove and fuel intervention contributed modestly to overall household energy use in rural China.

Journal article

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