Many festival goers must be familiar with the summer phenomenon. Are you dancing in a sweltering tent around two o’clock in the morning when it starts to rain – inside the tent. There is not a cloud in the sky outside.
The cause is clear: the sweat of the heated festival-goers evaporates, rises to the roof of the tent, until it condenses and falls as rain.
Radio station 3FM placed rain of sweat in a list of the biggest ‘festival horrors’. But festivals don’t seem to have a monopoly on sweaty rain: stories about nightclubs with a rainy season are also circulating on the internet. If summer is hot enough, sweat can sometimes fall in warm cafes. Every drop is accompanied by a cry of disgust.
How justified is that disgust? When undrinkable salty seawater evaporates and condenses again, relatively clean rainwater is created. By that logic, sweat rain should be very pure. How drinkable is sweat rain?
To start with, normal rainwater is not clean, says Jan Peter van der Hoek. He is professor of drinking water technology at TU Delft and innovation director at Waternet. “The moment seawater evaporates, for example, it will be quite clean. But the purity of rainwater is very much affected by the air it moves through. In agricultural areas you expect nitrogen compounds such as ammonia in the air, in urban areas with a lot of industry you will find more industrial contaminants.”
In this area, sweaty rainwater may still beat rainwater: in a festival tent it hardly has time to pick up contaminants in the air. “But it’s hard to say exactly what it will pick up,” says Van der Hoek. “What kind of contaminants do you find on the canvas on the ceiling of the festival tent?”
“Sweat in itself is not bad,” says Lisa Klous, sports scientist at TNO. She sends in a list with the composition of sweat, which she studied for her PhD research at the VU University Amsterdam. It consists of water with a lot of salts and minerals – which do not evaporate and remain on the skin – but also relatively large amounts of ammonia and urea. Urea is a relatively harmless waste product from the body, which mainly leaves the body through urine but also sweat.
Bacteria on the skin
“Urea and ammonia will partly evaporate with the sweat and end up in sweat,” says Van der Hoek. “And bacteria can easily grow on that, if it’s left for a while.”
For example, sweat itself is odorless. It only starts to smell when bacteria on the skin feed on urea and ammonia. When sweat rain descends on dancing festival-goers, it can also feed bacteria on the skin. So dirty, that rain of sweat, but only if you give bacteria on your skin the time to thrive.
Some festivals are now even taking measures, says Bente Bollmann, marketing manager of the Lowlands and Down The Rabbit Hole festivals. “The rain of sweat mainly occurred in the older ‘circus-type’ tents; our more modern tents do not suffer from it. Now ventilation is done in a different way. In recent years, perspiration has not occurred anymore.”