We all have too much stress in our lives. Stress from our job or school, our families, our social life. How about stress from traffic or public transportation? Or that neighbor's dog that barks all night?

There's another kind of stress that is silent, but it causes all kinds of trouble. It's called "oxidative stress" and it takes place in your cells.

To understand oxidative stress, you need to go back to high school chemistry. You may remember that different kinds of atoms (like hydrogen, oxygen, carbon, and so on) have different numbers of protons, neutrons and electrons. The number of protons equals the number of electrons, and protons and neutrons clump together to form the nucleus of an atom. The electrons sort of orbit around the nucleus, arrayed in different energy levels. Smaller atoms, such as hydrogen, only have one energy level; large atoms have several levels. Each energy level has a certain number of electrons it can hold. If the atom has the maximum number of electrons in its outermost layer, it is stable, non-reactive or inert. Helium, neon, argon, krypton, xenon and radon are inert  atoms.  

If there are fewer than the maximum number of electrons in the outermost energy level, the atom tends to "seek" other electrons.  Atoms will group together, or bond, if the number of electrons in their outermost levels complete a maximum number of electrons. Groups of atoms bonded together are molecules.

Breaking bonds

When some molecules interact with certain other molecules, they may have a chemical reaction. The bonds between electrons in the original molecules are broken and new bonds are formed with the new molecules, causing changes in their physical and chemical properties. There are all kinds of chemical reactions—fires and explosions come to mind—but one of the most common reactions is oxidation.

In chemical reactions, there is always a balance of the loss and gain of electrons. Oxidation occurs when a molecule accepts electrons (whether oxygen is involved in the reaction or not), and the original molecule is the oxidant. The molecule that transfers or donates the electrons is said to be reduced, and may be called the reductant, or sometimes an antioxidant. The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair.

Oxidation and reduction

Oxidation can be as simple as rusting—in which iron molecules react with oxygen molecules to recombine as ferrous oxide—which is more "stable" than either oxygen or iron. Oxidation can also take place with large, complex molecules.

In the very large molecules that make up living things (like humans), oxidation can be caused by normal processes such as digestion, respiration or exercise. Oxidation also occurs because of outside influences such as the sun's rays, pollutants, other chemicals, and so on. When these molecules lose electrons to become oxidants, they become unstable and eventually the cell (or whatever they are part of) can be damaged. Sometimes the unstable molecules are called free radicals. Fortunately, there are other molecules, the reductants or antioxidants, that can donate electrons, returning stability to the molecule.

The body tends to keep a balance between the molecules that lose electrons and those that donate them back. But sometimes there are just too many causes for the molecules to lose electrons and not enough of the donor molecules. This results in an overabundance of "oxidized" molecules, leading to oxidative stress.