1) I would immediately be skeptical of The Daily Mail, it's the Fox News of the UK.

2) They don't name or link the specific study and a few minutes of searching on Google Scholar didn't find such a study by a Carmen de Jong. I did find her entry on Artificial Production of Snow in the Encyclopedia of Snow, Ice and Glaciers, 2011.

3) I found what looks like a college term paper (not original research) that cites the SAME DAILY MAIL ARTICLE without citing actual articles by Carmen de Jong. Most of the paper is concerned with water pH, salinity, and the effect on plantlife and soils, the reference to human impact is not central.

4) There is a french ENSP paper that cites actual studies (NIOSH did one in New Mexico in the 90s), but I couldn't find them online. The citations summarize the findings as being generally similar to what cotton-industry workers get, except the exposure levels are about 100 times lower. Some people are hypersensitive to the "endotoxins" in the dead bacteria. If you stick your face right in the snowcannon you get exposed to about 70x the level of endotoxin as in normal snow.

5) They tried to do some epidemiology with surveys of ski hill workers, especially those who work in snowmaking. They couldn't get enough data to draw conclusions.

https://documentation.ehesp.fr/memoires/2005/ase_igs/5_snomax. pdf

good fact check

I will consider spending more time reading the Encyclopedia of Snow, Ice, and Glaciers

I was procrastinating at work, and I ski a lot so I was personally invested in finding out. Plus I hate the Daily Mail.

TLDR; Don't work in a cotton mill.