Chlorine in swimming pools
Is Chlorine in Swimming Pools Safe? by The New York Times overviews that chlorine is probably safe although it can be mixed with urine to produce toxic chemicals.
Mark Rober had a video talking about this How to measure HOW MUCH PEE IS IN YOUR POOL where he showed the statistics about swimmers with Asthma. However, this statistics is probably an example of faulty Causal inference. It’s probably not because swimmers develop Asthma due to the chemicals, but because those who with asthma are more likely to choose Swimming instead of other sports.