The amount of trichloramine can be measured, but the tools used to do so aren’t generally available at pools. And cyanogen chloride is even harder to measure. “[It’s] a very dynamic chemical. It forms very rapidly but it also decays very rapidly, and it’s quite volatile,” says Blatchley. What that means, he says, is that once it forms in a pool, it doesn’t stick around very long. Of the two substances, Blatchley says cyanogen chloride is the most concerning. It’s a toxic substance, and once it reaches a high enough level, it can be detrimental to human health. But the problem is that because it forms and decays so quickly, it’s very hard to figure out how high those levels in a pool can get.