Analyzing the effects of drought at different time scales on cause-specific mortality in South Africa

Abstract

South Africa (SA) is highly vulnerable to the effects of drought on the environment, economy, and society. However, its effect on human health remains unclear. Understanding the mortality risk associated with different types of droughts in different population groups and by specific causes would help clarify the potential mechanisms involved. The study aims to comprehensively assess the effect of droughts of varying time scales on cause-specific mortality (all; infectious and parasitic; endocrine, nutritional, and metabolic; cardiovascular; respiratory) in SA (from 2009-2016) and identify more vulnerable profiles based on sex and age. We also evaluated the urbanicity and district-level socioeconomic deprivation as potential risk modifiers. We used a two-stage time-series study design, with the weekly standardized precipitation-evapotranspiration index (SPEI) calculated at 1, 6, 12, and 15 months of accumulation to identify droughts of different duration (SPEI1, 6, 12, 15, respectively). We applied a quasi-Poisson regression adjusted by mean temperature to assess the association between each type of drought and weekly mortality in all district municipalities of SA, and then pooled the estimates in a meta-regression model. We reported relative risks (RRs) for one unit increase of drought severity. Overall, we found a positive association between droughts (regardless the time scale) and all causes of death analyzed. The strongest associations were found for the drought events more prolonged (RR [95%CI]: 1.027 [1.018, 1.036] (SPEI1); 1.035 [1.021, 1.050] (SPEI6); 1.033 [1.008, 1.058] (SPEI12); 1.098 [1.068, 1.129] (SPEI15)) and respiratory mortality (RRs varied from 1.037 [1.021, 1.053] (SPEI1) to 1.189 [1.14, 1.241] (SPEI15)). An indication of greater vulnerability was found in younger adults for the shortest droughts, in older adults for medium-term and long-term droughts, and children for very long-term droughts. However, differences were not significant. Further evidence of the relevance of urbanicity and demographic and socioeconomic conditions as potential risk modifiers is needed.

Keywords: SPEI; South Africa; cause-specific mortality; drought; vulnerability assessment.

Figure 1. Diagram of the design and procedures conducted in this research study.

Tmax: maximum temperature registered each day; Tmin: minimum temperature registered each day; Tmean: mean 24-hourly temperature within each day; Prec.: total precipitation accumulated each day. SPEI: the Standardized Precipitation Evapotranspiration Index calculated at 1, 6, 12, and 15 months of accumulation (SPEI1; SPEI6; SPEI12; SPEI15, respectively). A00–B99: mortality due to certain infectious and parasitic diseases; E00–E90: mortality due to endocrine, nutritional, and metabolic diseases; I00–I99: mortality due to circulatory diseases; J00–J99: mortality due to respiratory diseases.

Figure 2. Pooled results for drought-mortality associations categorized by groups of sex, age and causes of death (relative risk, RR) using various drought metrics to characterize short, medium, long, and very long term (SPEI1, 6, 12, 15, respectively) in South Africa between 2009 and 2016.

Estimates were calculated for one unit increase of drought severity. Horizontal bars correspond to the 95% confidence intervals. All: all-cause mortality; A00–B99: mortality due to certain parasitic and infectious diseases; E00–E90: mortality due to endocrine, nutritional, and metabolic diseases; I00–I99: mortality due to circulatory diseases; J00–J99: mortality due to respiratory diseases.

Figure 3

Map risk (RR, 95%CI) of all-cause mortality associated with drought measured by the weekly SPEI calculated at different time scales (1, 6, 12, 15 accumulation months; SPEI1, 6, 12, 15) in the total population at district municipality level in South Africa between 2009 and 2016 (represented in a color gradient).