Open water swimming is a popular pastime in the UK, particularly during the hotter summer months. However, a recent spate of tragedies in England and Scotland have prompted warnings from rescue services, urging people to understand the risks posed by swimming in open water.

The activity is used to describe swimming in rivers, lakes, natural pools and the sea. Many people swim in these places recreationally, but it is important to understand how to do so safely.

According to the National Water Safety Forum, an estimated 40 people have drowned in the UK since the heatwave began on 14 July. This is triple the normal rate of water deaths, which is 19 per year on average.

Some of the deaths included a 19-year-old man in Lancashire, who died near St Anne’s Pier, and a boy aged 16 died in Scotland at Loch Lomond.

A senior officer for the Scottish Fire and Rescue Service said on Monday that the past weekend was one of the worst in memory for the fire service, after six people died in Scotland’s waters – half of whom were young boys under the age of 15.

Meanwhile, Tony Watson, head of visitor services and communications at the Lake District national park, said there has been an increase in people swimming while drunk, which “really reduces people’s common sense and fear factor”.

He added: “We might be having Mediterranean air temperatures but we’ve still got traditionally cold northern English waters.”

What are the risks of open water swimming?

Cold water is one of the biggest risks of open water swimming if a person is unfamiliar with the activity. Cold water shock can occur when they are suddenly immersed in the cold water, and is considered a principal underlying factor in a drowning death.

When the body is suddenly immersed in cold water, closed blood vessels in the skin can result in the heart working harder to pump blood throughout the body, which leads to rising blood pressure.

At the same time, people instinctively gasp when in cold water, which means the ability to keep controlled steady breathing is lost for a while.

Both of these can lead to a sense of panic, inhalation of water, and in some circumstances, cardiac arrest, says the National Water Safety Forum.

To avoid cold water shock, swimmers are advised to wear wetsuits and allow their body to acclimatise to the change in temperature, instead of jumping straight in.

Another factor people should take into account before they go swimming in open water is the location. The safest places to swim are supervised beaches with lifeguards and outdoor pools. These areas are regularly checked, managed and often used by swimming, triathlon and lifesaving clubs.

The Environment Agency has a list of designated bathing waters in the UK at which water quality and risks of pollution are assessed, but many of these sites are not supervised.

Away from these sites, it can become difficult to assess how safe it is to swim. People should be especially cautious when considering a swim in quarries and reservoirs, which can be deceptively deep and cold, rivers with weirs and fast flowing water, and potentially polluted locations near rainwater run-offs, livestock and estuarial waters.

The Royal National Lifeboat Institution also recommends that people check the weather forecast and sea conditions before a swim. A potential danger of swimming in the sea is getting caught in a strong current. Swimmers are advised: “If in doubt, don’t go out.”

Lee Heard, UK charity director of the Royal Life Saving Society, said: “Whilst we recognise how tempting it is to cool off in the UK’s beautiful waterways, they hide hazards that tragically take lives each year and we urge the public to use caution when entering the water, getting acclimatised to the water temperature before jumping in.

“The difference between the air temperature and water temperature can literally take your breath away; this is called cold water shock. It is silent, invisible and deadly.”

Additional reporting by PA