For those who want lots of detail about toxicity of pyro, have a look at the site address below. Note that it's an ingredient in many common hair dyes and occurs in smoked meats (ham etc). One needs to be careful of all chemicals, including those in our food.
One of the things that I like about PMK is that I have to mix stock solution only once every two or three years. Having said that, I'm moving over slowly to pyrocat-HD mainly because of its ability to be used more dilute for minimal agitation, and I like that effect very much.