How do heat health alerts work?

The weather alert service warns the public in England when high or low temperatures could damage their health.

The system is run by the UK Health Security Agency and the Met Office.

It includes both heat health and cold health alerts

Heat health alerts are issued between 1 June and 30 September, and cold health alerts are published between 1 November and 30 March.

As well as warning the public, the system sends guidance directly to NHS England, the government and healthcare professionals during periods of adverse weather.

Alerts are categorised according to severity and include:

• headline weather conditions expected in the coming days
• details of how weather conditions will affect each region
• links to additional information, advice and guidance

The system was designed to help reduce illness and deaths during periods of extreme weather.

• See current weather-health alerts
• Check the UK weather warnings in place
• When is a heatwave 'really' a heatwave?

The level of alert is based on Met Office forecasts and data.

There are four levels ranging from green (least severe) to red (most severe):

Green

Green is the normal level, when advice is given on how people should prepare to respond if temperatures rise or fall.

Yellow

Yellow alerts are issued during periods of hot or cold weather that are only likely to affect those who are particularly vulnerable, for example the elderly, or those with existing health conditions.

Amber

Amber alerts are issued in situations that could potentially put the whole population at risk. The NHS may see increased demand on GPs and ambulances, for example.

Travel disruption is also likely.

Red

A red alert is the most severe.

It is issued in situations when hot or cold weather would be a significant risk to life, even for the healthy population, and could lead to failures of critical national infrastructure, such as power outages or roads and rail lines being closed.

As the body gets hotter, blood vessels open up. This leads to lower blood pressure, and makes the heart work harder to push the blood around the body.

This can cause mild symptoms such as an itchy heat rash or swollen feet, as blood vessels become leaky.