BACKGROUND: Heatwaves can lead to increased mortality. In the Portuguese heat-health warning system (HHWS), ÍCARO, a daily report with heat-related mortality prediction is sent to heat-health action plan (HHAP) practitioners. HHAP practitioners assess risk and implement measures to prevent heatwave-related impact, but ÍCARO's use and understanding are unknown. We assessed ÍCARO's use and understanding by key HH...