What is a Level 4 heat-health warning and why has a national emergency been triggered?

The first ever red warning for extreme heat has been issued by the Met Office, posing a potential risk to life.

Temperatures are predicted to reach 40C in London, and there's a very high chance we will see the hottest day on record for the UK - beating the existing high of 38.7C in Cambridge in 2019.

In another first, the UK Health Security Agency increased its heat health warning from level three to level four - triggering a national emergency.

Here's what Level 4 means and how it's decided.

What is Level 4?

When a heatwave is so severe or prolonged, the impact can be far-reaching, leaving the health and social care system feeling the strain.

"At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups," the UKHSA said.

At Level 4, even the fit and healthy could fall ill or die, not just the high-risk and vulnerable groups.

During severe heatwaves that last for a considerable period of time, transport, food, water, energy supplies, businesses and health and social care services can all be impacted.

Who or what decides that we are at Level 4?

The decision to go to Level 4 is made across government and coordinated by the Civil Contingencies Secretariat – a department within the Cabinet Office responsible for planning for emergencies.

The response, according to the UK Health Security Agency (UKHSA), would likely involve:

• government departments

• executive agencies

• the public sector, including the health sector

• volunteers

While there isn’t a set temperature to trigger a Level 4 response, there are threshold temperatures for each region that form part of the consideration.

What are the temperature thresholds?

• London: 32C (18C at night)

• South-east: 31C (16C at night)

• South-west: 30C (15C at night)

• Eastern: 30C (15C at night)

• West Midlands: 30C (15C at night)

• East Midlands: 30C (15C at night)

• North-west: 30C (15C at night)

• Yorkshire and Humber: 29C (15C at night)

• North-east: 28C (15C at night)

What could change?

• Transport infrastructure

Road surfaces could melt, which is most likely to happen when in direct sunlight as the surface temperature may not be dependent on the air temperature.

According to government analysis, tarmac can start to melt at 33C.

Any traffic congestion could then lead to delays on motorways or main roads, which could have potentially serious consequences for anyone stranded in vehicles, especially the elderly or young children.

Parts of the railway lines could buckle, though this also depends on the maintenance status of the track.

The UKHSA advises preventative measures when air temperatures reach 22C.

The most extreme precautions would only kick in at air temperatures of 36C, which is likely to give a railhead temperature of over 50C.

On the London Underground, plans could be put in place to provide a supply of bottled water, if temperatures forecast do not to fall below 24C for three days running.

• Workplaces and schools

In offices or similar environments, such as schools, the temperature in workplaces must be "reasonable", according to the Health & Safety Executive. But there's no law for a maximum working temperature, or when it's too hot to work.

Employers, however, must ensure they are abiding by health and safety laws, including keeping the temperature at a comfortable level for staff, and providing clean and fresh air.

Individual schools may chose to relax their uniform policy in scorching heat, or even send students home early, provided sufficient notice is given to parents.

In the heatwave of 2006, dozens of schools in England closed or sent pupils home early because of soaring temperatures.

Some schools, with poor ventilation and little air-conditioning, were reported to have recorded temperatures of more than 36C in many classrooms.

• Power supplies

Rising temperatures increase the demand for energy as more begin to use air-conditioning units and fans – all at a time when suppliers typically try to save power for the winter.

The heat can also cut the power-carrying capacity of the system, as it becomes harder to cool conductors.

High air temperatures pose a problem as nuclear reactors can trip out at above 40C, although this has never happened at any site (as the highest temperature recorded in England has been 38.7°C).

Generally, the rising temperatures lower power station efficiency.

• Water quality and shortages

Water companies have plans in place to deal with failure in the supply of mains water or sewerage services, UKHSA says.

These plans are “regularly reviewed and tested” and independently certified every year.

If the country does get hit by a shortage, water saving measures – such as reducing water pressure or limiting 24/7 supply – would be rolled out.

If the water supply is lost, suppliers are required to provide water through alternative means, such as in static tanks or bottled water.

Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know

Companies must give no fewer than ten litres per person per day, with special attention given to vulnerable people, hospitals and schools.

If piped water supply is interrupted for more than five days, the minimum requirement jumps to 20 litres per person per day.

Strong demand during a heatwave has the potential to jeopardise the availability of water supplies, particularly in southern parts of the UK.

This can lead to local hose-pipe restrictions if high temperatures persist.

In lasting hot temperatures, blue-green algae can also grow more quickly, which can cause problems for animals, including fish.

A prolonged heatwave may increase odour and dust and even spark a vermin infestation.

The UKHSA says additional measures would be needed to mitigate these problems, including more frequent refuse collections and pollution control measures at landfills and other waste sites.

• Potential wildfires

Grass, trees and bushes will be drier than usual, risking wildfires spreading in rural parts.

• Crops

During a heatwave, crops may not be harvested at appropriate times and may be lost or quality and nutritional value may be reduced.

Flowering and pollination may also be affected, reducing fruit and grain yield.