Protozoa are a group of single-celled microorganisms. Larger and more complex than bacteria, they can nevertheless easily be ingested along with pool water. Some species, such as
Giardia lamblia and
Cryptosporidium parvum, can be 15,000 to 1 million times more resistant to chlorine than most
algae and
bacteria. Thus, constant circulation and a high
chlorine residual are crucial to keeping these parasites under control.
