The NWS highlights that extreme heat poses significant health risks, particularly to vulnerable populations such as the elderly, children, and those without access to adequate cooling or hydration. During 'Major' and 'Extreme' HeatRisk days, the entire population is at risk due to prolonged high temperatures and minimal overnight cooling. Data from Maricopa County shows a direct correlation between HeatRisk levels and spikes in emergency room visits for heat-related illnesses. Even on 'Low' risk days, heat can still pose a threat to sensitive groups. (Source: NWS Phoenix Heat Page, Source time: not specified)