Urban heat exposure and outdoor occupational heat-related illnesses in Singapore – A nationwide time-stratified case crossover study

Background Urban Heat Island (UHI) effects pose a threat to workers in outdoor occupational environments. Few studies have evaluated the association between heat exposure and outdoor occupational heat-related illnesses (OOHRIs) in tropical urban settings. We examined how ambient temperature and humidity influence outdoor occupational HRIs in Singapore, a tropical city-state. Methods We included all notified OOHRIs from 2009 to 2023 and contemporaneous 24-hourly averages of ambient temperature and absolute humidity. Using a time-stratified case-crossover study design, referencing 4 control days from two preceding and two subsequent weeks, we examined heat exposure and OOHRIs using conditional logistic regression. Results We included 40 reported cases of OOHRIs. A 1 °C increase in ambient temperature was associated with a 2.5-fold (OR: 2.53, 95% CI: 1.25 to 5.12) increase in outdoor occupational HRI on the same day. A sustained 1 °C increase in ambient temperature over the present and previous 2 days was associated with a cumulative 3.8-fold (OR: 3.83, 95% CI: 1.59 to 9.22) increase in the odds of OOHRI. A 1 g/m 3 increase in absolute humidity sustained over the present and previous 4 days was associated with a 2.8-fold (OR: 2.79, 95% CI: 1.03 to 7.59) increase in the odds of OOHRI. Non-Singaporeans appeared to experience higher odds of temperature-driven outdoor HRI compared to locals. Conclusion Ambient temperature and absolute humidity were important urban climate-related drivers of OOHRIs. Heat mitigation measures in outdoor workplaces ought to be strengthened and tailored to mitigate UHI effects to preserve workers' health.