It talks about, demonstrates and measures the air quality around vinyl. Apparently the things are leaching dangerous compounds into the air constantly, especially when being played. It made his air quality measuring device give an alarm, instead of just reporting the number of particles or something.
I'm not sure, maybe it's just one of those thousands of every day life things which are technically carcinogenic, or not. It made me pause personally though, having recently lost a friend, who worked at a record store, died at a young age to cancer, had otherwise always been a strong healthy man. I'm actually more comfortable talking about this here, semi-anonymously, I don't think it'd be wise to bring this up IRL with anyone, on account of it possibly not having to do with anything. I kind of half-wish I hadn't seen the video.