Cause-specific hospital admissions on hot days in Sydney, Australia

Abstract

Background: While morbidity outcomes for major disease categories during extreme heat have received increasing research attention, there has been very limited investigation at the level of specific disease subcategories.

Methodology/principal findings: We analyzed daily hospital admissions for cardiovascular (CVD), respiratory (RD), genitourinary (GU) and mental diseases (MD), diabetes (DIA), dehydration (DEH) and 'the effects of heat and light' (HEAT) in Sydney between 1991 and 2009. We further investigated the sensitivity to heat of subcategories within the major disease groups. We defined hot days as those with temperatures in the 95(th) and 99(th) percentiles within the study period. We applied time-stratified case-crossover analysis to compare the hospital admissions on hot days with those on non-hot days matched by day of the week. We calculated the odds ratios (OR) of admissions between the two types of days, accounting for other environmental variables (relative humidity, ozone and particulate matter) and non-environmental trends (public and school holidays). On hot days, hospital admissions increased for all major categories except GU. This increase was not shared homogeneously across all diseases within a major category: within RD, only 'other diseases of the respiratory system' (includes pleurisy or empyema) increased significantly, while admissions for asthma decreased. Within MD, hospital admissions increased only for psychoses. Admissions due to some major categories increased one to three days after a hot day (e.g., DIA, RD and CVD) and on two and three consecutive days (e.g., HEAT and RD).

Conclusions/significance: High ambient temperatures were associated with increased hospital admissions for several disease categories, with some within-category variation. Future analyses should focus on subgroups within broad disease categories to pinpoint medical conditions most affected by ambient heat.

Figure 1. Location of the Sydney Statistical Division.

Figure 2. Odds ratios (adjusted for relative humidity, O₃ and PM₁₀) comparing hospital admissions due to several specific diseases between extremely hot days and control days in the Sydney Statistical Division between July 1^st, 1991 and June 30^th, 2009; on a hot day and 1, 2, and 3 days after the hot day at the 95^th percentile of average temperature (results shown after the FDR adjustment). Note: the x-axis scale for admissions on hot days is different.

Figure 3. Odds ratios (adjusted for relative humidity, O₃ and PM₁₀) comparing hospital admissions due to several specific diseases between extremely hot days and control days in the Sydney Statistical Division between July 1^st, 1991 and June 30^th, 2009; on a hot day and 1, 2, and 3 days after the hot day at the 99^th percentile of average temperature (results shown after the FDR adjustment). Note: the x-axis scale for admissions on hot days is different.