Citric acid combines with chlorine to form toxic trihalomethanes. It's not clear whether this is an issue on outdoor pools, but since ascorbic acid works better, is cheaper (when purchases as ascorbic acid rather than pool chemical) and doesn't have this issue with toxic byproducts . . . citric acid ends up on the DO-NOT-USE list.

Plus, it irritates me no end that Natural Chemistry doesn't provide users with enough info to use the products effectively.