Many commercial pizza flours contain dough enhancers, relaxers and
extenders. They offer different modifications to the taste and
handling qualities of the dough. Domestic pizza flours may contain
some or none of these additives. This is not to say that pure
(wheat-only) flours are inferior for pizza, it's not as simple as
that; in fact, it's too complex to go into detail here. One of the
relaxers that turns some people off is L-cysteine hydrochloride also
known as E920 or E921. This product is often made from hydrolized
human hair (don't believe me? search the internet!). Minute
quantities in the range of tens of parts per million help soften the
dough and thus reduce processing time.
If additives bother you, don't use any commercial (that is, for
professional bakers and pizzerias) pizza flour. There are many
alternatives that give excellent results, but you may want to closely
check the ingredients on the manufacturer's web site before committing.