Oxidative stress describes an imbalance between cells' production and elimination of byproducts of the breakdown of oxygen. These byproducts, known as reactive oxygen species (ROSs), are important for cell functioning but cause damage in high amounts. ROS belong to a larger category of highly reactive chemicals called free radicals. Because cells need ROSs to function, some researchers have described oxidative stress as a good thing in some contexts. Others say that oxidative stress, by definition, is bad.

Oxidative damage is involved in aging and in several diseases. Chemicals that inhibit oxidation and the production of ROSs are called antioxidants. Some research suggests antioxidants could help limit oxidative damage. However, it's unclear how much or to what effect.

What causes oxidative stress?

ROSs are produced naturally when the body breaks down diatomic oxygen (O2) as part of cellular respiration, the process of extracting energy from glucose (sugar). In organisms that have one, this happens in the mitochondria, the so-called powerhouse of the cell. As part of the last stage of cellular respiration, the cell separates electrons, or negative subatomic particles, from byproducts of glucose. This enables the cell to make a molecule called adenosine triphosphate (ATP), its main power source. The cell needs oxygen to accept the electrons at the end of this process, and most oxygen molecules are finally transformed into water.

However, some oxygen molecules receive fewer electrons and are instead transformed into free radicals, specifically ROSs. These substances' missing electrons are what make them extremely reactive, and they will react with many substances in the cell to gain electrons and become more chemically stable. Some common ROSs are peroxides (such as hydrogen peroxide), superoxide and the hydroxyl radical.

"We constantly make those reactive oxygen species, as we call it, in every cell of the body," Ursula Jakob, a professor of molecular, cellular and developmental biology at the University of Michigan, told Live Science. In addition to being a byproduct of respiration, ROSs are used in cell signaling, or sending messages within or across cells.