With Europe already reeling from extreme heat, a dangerous heatwave is set to grip large parts of the United States over the July 4 holiday weekend. The National Oceanic and Atmospheric Administration’s forecasts indicate that nearly 100 million people could be exposed to the highest level of heat risk.
Predictions from the US National Weather Service’s HeatRisk system show the most severe conditions stretching from the Midwest through the Ohio Valley and into the Northeast. On July 3, about 96 million people are forecast to fall under the “extreme” risk category, the NOAA’s highest warning level. The areas at greatest risk include parts of Ohio, Kentucky, West Virginia, Virginia, Pennsylvania, and New York.
It is being driven by a powerful heat dome parked over the eastern half of the country. According to Reuters, daytime temperatures are expected to reach 35–37°C across many cities, while humidity could push the heat index above 38°C. Forecasters expect the most intense conditions between July 2 and 5, coinciding with one of the busiest travel weekends of the year.
The biggest concern is not just the daytime heat but the lack of overnight relief. The NOAA considers unusually warm nights and the duration, rather than temperature alone. In several eastern cities, temperatures are expected to remain above 27°C overnight, increasing the risk of heat-related illness, especially among older adults, children, and people with pre-existing health conditions.
The warning comes as scientists increasingly link extreme heat to illness and death. The latest Lancet Countdown report found that Americans experienced nearly 14.1 heatwave days in 2024, and 72 per cent of those days would not have occurred without climate change. Heat exposure also resulted in the loss of more than 3.5 billion potential labour hours, with 45 per cent of these hours lost in the construction sector alone. The heat also carried an economic cost, with an estimated $122 billion in potential income losses in 2024.
Heat may seem like a temporary weather event, but its impact is measurable. Data on heat-attributable mortality show that heat accounted for 0.57 per cent of all deaths globally in 1990. By 2010, that figure had risen to 1.37 per cent, one of the highest levels recorded in the past three decades. Europe has experienced some of the sharpest increases, with major heatwave years such as 2003 and 2010 leaving a clear mark on mortality.
According to the World Health Organisation, excessive heat causes nearly half a million deaths worldwide each year. As millions of Americans prepare for Independence Day celebrations, forecasters say the greatest danger may not come from a single hot afternoon, but from several consecutive days of intense heat and warm nights that give the body little chance to recover.
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