Hydrogen peroxide in the human body

Hydrogen peroxide (H 2 O 2 ) is widely regarded as a cytotoxic agent whose levels must be minimized by the action of antioxidant defence enzymes. In fact, H 2 O 2 is poorly reactive in the absence of transition metal ions. Exposure of certain human tissues to H 2 O 2 may be greater than is commonly supposed: substantial amounts of H 2 O 2 can be present in beverages commonly drunk (especially instant coffee), in freshly voided human urine, and in exhaled air. Levels of H 2 O 2 in the human body may be controlled not only by catabolism but also by excretion, and H 2 O 2 could play a role in the regulation of renal function and as an antibacterial agent in the urine. Urinary H 2 O 2 levels are influenced by diet, but under certain conditions might be a valuable biomarker of ‘oxidative stress’.