And Dr Tom Makin claims you shouldn’t re-enter the bathroom for another 20 minutes after that.
When you arrive back from holiday, often after a long journey home, the first thing you might think of is a long, hot shower.
But you might want to wait a bit before stepping beneath those steaming jets of water, if you want to take the advice of a microbiologist
Rather than getting straight in, Dr Tom Makin will let the shower run for 20 minutes. Even then, he won’t even re-enter the bathroom for another 20 minutes.
The reason, he says, is to prevent lethal bacteria causing life-threatening conditions.
Dr Makin says that in the time a person is on holiday, stagnant water in your shower head could turn toxic.
This means people are at greater risk of inhaling bacteria called legionella.
Legionella can cause pneumonia and organ failure, both of which can be fatal.
Speaking to the Mail on Sunday , Dr Makin said: “I cover my nose and mouth with one hand and turn on the shower with the other.
“After 20 minutes or so, I’ll turn the shower off and then I won’t go back into the bathroom for at least another 20 minutes.”
Going away in the summer increases the risk, because legionella thrives when the temperature is between 20C and 45C.
Garden hoses and sprinkler systems are also said to be potentially risky.
The World Health Organisation says that legionella is on the rise, in likelihood because people are taking showers rather than baths.
An estimated 500 Brits contract the condition each year.