Body Heat

At the Royal commission in 1919 a medical witness said he had visited a seam where the temperature was over 90 degrees. He took the temperature and pulse rate of the miners' during a meal break, even resting both was over 100 in every case.

Of Tilmanstone colliery:

'My father often told us how the men who wore only briefs and boots perspired in the terrific heat. One day out walking with him we passed one of the outlets from the mine with water gushing out and I asked if it was the miners' sweat.'

(I think the little girl was mislead about the briefs, 1920's underwear was large coarse and woolly in the pictures I've seen the men are wearing only boots,( cap- I think not a helmet?) and a leather tool belt)
We are not told which pit the medical witness visited, but I'm betting Snowdon was both hotter and more humid.

Sweating is a less effective method of cooling when humidity is high, if the body cannot losef excess heat the core temperature rises. the ability to concentrate is lost, (accident rate increases) usually irritability increases (murder rate goes up in a heat wave), there's nausea, al loss of the desire to drink ( that could be fatal- miners needed to drink about 24 pints of water in snowdon colliery) In the short term this could lead to heat cramps, rashes , heat exhaustion and stroke. In the long term it's hard to say, so few men today do hard physical labour under these conditions.
Snowdon miners took salt tablets. Getting the balance of water to salt intake was essential, too much water , too little salt could cause hypernatria(Sodium imbalance)

Long term exposure to excessive heat is thought to cause infertility, damage to brain chemistry and the nervous system, increased susceptibility to diseases especially kidney, liver, heart digestive system and skin. U.S. Military records suggest that soldiers who have had one incident of heat stroke are more likely to have mental difficulties, to drop out early and so on.

Even leaving aside the devastating and better known effects of coal dust on the lungs or of darkness on the eyes, forgetting the social factors: it's just remarkable that men lived and worked in such conditions- and given the association between heat irritability and violence- it's remarkable how peaceable Aylesham was.