Seattle had a record-breaking hot spell last week. At 30C (86F), the city by the Canadian border was hotter than Death Valley in Arizona. However, authorities advised anyone going out on the water of a lingering threat from cold if they fell in.

“If you survive long enough to get hypothermia, you’ve done well,” warns the Washington state parks advice on cold water immersion. “Most drown at the first stage and in the first few minutes from cold water shock.”

Water heats up slowly compared with air. However much sun there is, rivers fed by meltwater are still bitterly cold; some of those around Seattle do not rise above 15C at any point in summer. Falling into water at this temperature can produce cold water shock, which increases breathing rate and causes involuntary gasping for breath, often resulting in water being inhaled. The shock also means people can only hold their breath for a few seconds. In addition, the condition may bring on a feeling of panic, further increasing the risk of drowning.

Another effect of cold water shock is constricting the blood vessels, making the heart work harder and sometimes triggering a heart attack.

The official advice is to enjoy the weather, but be aware and always wear a lifejacket while on the water.