People with lung conditions, heart problems and the elderly are being told to avoid strenuous physical activity as air pollution reaches its highest level.

The UK Met Office has triggered a health alert after forecasting maximum levels of air pollution across large parts of the south of England including Medway and Kent.

It comes after motorists woke up to find a thin coating of red dust on their cars on Monday which had been transported from the Sahara Desert.

According to the Daily Air Quality Index, residents in parts of the south coast, South Wales, Somerset, Chilterns and the East Midlands are being advised to “reduce physical exertion”, particularly outdoors, today and tomorrow.

In addition, adults and children with lung problems, adults with heart problems, and older people, should avoid all strenuous physical activity until the 24-hour pollution peak subsides on Thursday.

People with asthma are also advised that they may find they need to use their reliever inhaler more often.

The Daily Air Quality Index (DAQI) is the standard index defined by Defra for characterising air quality across the UK.

It provides health advice in the form of recommended actions that should be taken, according to the level of air pollution. The index is numbered one to 10 with one being low and 10 very high.

The Met Office has issued a level 10 forecast for the south coast of England from the tip of the Kent coast along to Dorset and up into South Wales and across Nottinghamshire and Derbyshire.

Strood GP Dr Julian Spinks advised people to be extra careful.

He said: "At the moment we have a triple whammy: we have high pollen levels, high pollution levels and also this sand that has come over.

"The problem with that is it can get deep down into people's lungs when they're outside.

"For most people who don't have chronic diseases it may not make any difference.

"However, if you have things like asthma, lung and severe heart disease, going out and breathing a lot of the pollution and dust that's in the air can trigger spasm in the airways giving you breathing difficulties and perhaps even a full-blown asthma attack.

"I think people know that pollution is a problem but I don't think many people realise how quite badly it can be for health.

We're at a level which is near the top severity for pollution and that does have a major impact. It does mean that people are restricted, they can't go out like they would normally."

Residents around Kent and Medway were among people across the south of Britain who have been affected by the Sahara dust phenomenon.

It was caused by an uncommon weather pattern, where small specks of sands were picked up from the Sahara Desert and transported north through Europe.

A number of elements created the right condition for it to happen – a sandstorm in the Sahara region, the wind and a certain type of rain, which brought it down and when it dried left a residue on the cars.

As well as the south east, drivers in Devon and Cornwall also received a dusting.