How many harmful reactive oxygen species exist in the body?
There are 6 common harmful reactive oxygen or nitrogen species existing in the body. They are peroxyl radical (ROO.), hydroxyl radical (HO"), hydrogen peroxide (H2O2.), superoxide ion (O2.), singlet oxygen (1O2) and peroxynitrite (OONO-)1

1 Halliwell, B. Reactive Oxygen Species in Living Systems - Source, Biochemistry, and Role in Human-Disease. Am. J. Med. 1991, 91, S14-S22.