Free radicals are produced by your own body every second of every day, and they also flood in from outside sources like sunlight, pollution, and food. They form when a molecule loses or gains an electron, leaving it with an unpaired electron that makes it highly reactive. This instability drives the molecule to steal electrons from nearby cells, proteins, or DNA, setting off chain reactions that can damage tissue. Understanding where they come from helps explain why some are unavoidable, some are actually useful, and some you can minimize.
Your Mitochondria Produce Them Constantly
The single largest source of free radicals in your body is your own energy production. Inside nearly every cell, mitochondria convert food and oxygen into usable energy. This process involves shuttling electrons through a chain of molecular complexes, and it’s not perfectly efficient. At certain points, electrons slip off track and latch onto nearby oxygen molecules instead of continuing down the chain. The result is superoxide, one of the most common free radicals in biology.
The leak happens at a specific site: a molecule called flavin in the first complex of the chain. When flavin is fully loaded with electrons and its binding site is momentarily empty, an oxygen molecule can swoop in and grab one of those electrons. The rate of superoxide production depends on how much oxygen is available and how many of these flavin molecules are in that vulnerable, fully reduced state. This isn’t a malfunction. It’s a built-in imperfection in a system that runs trillions of times per day across your cells. Under normal conditions your body neutralizes most of these escaped radicals with antioxidant enzymes, but the sheer volume means some always slip through.
Your Immune System Makes Them on Purpose
Not all free radicals are accidental byproducts. Your white blood cells deliberately manufacture them as weapons. When immune cells called phagocytes detect a bacterium or other pathogen, they engulf it and then unleash what’s known as a respiratory burst: a rapid spike in oxygen consumption that generates a flood of superoxide inside the compartment trapping the invader.
This burst is powered by an enzyme complex that assembles quickly on the cell membrane once the immune cell is activated. Superoxide is just the opening salvo. It rapidly converts into hydrogen peroxide, which is then combined with other chemicals the cell produces to create even more destructive species, including hypochlorous acid (essentially bleach) and hydroxyl radicals. These are potent enough to tear apart bacterial membranes and proteins. The system is remarkably effective, but it’s also indiscriminate. Some of those reactive molecules escape the compartment and damage surrounding healthy tissue, which is part of why chronic inflammation causes long-term harm.
Ultraviolet Light Triggers Them in Your Skin
Sunlight, particularly UVA rays (320 to 380 nm), is one of the most potent external triggers of free radical production. When UV photons penetrate your skin, they transfer energy to molecules inside your cells, knocking electrons loose and generating reactive oxygen species directly in the tissue. UVB rays (290 to 320 nm) do the same, though they penetrate less deeply.
Your skin cells have a built-in defense: glutathione, a molecule that absorbs and neutralizes these radicals before they cause lasting damage. But under heavy or prolonged sun exposure, that defense gets overwhelmed. When free radicals damage cell membranes faster than they can be repaired, skin cells activate stress-response genes. One of these triggers increased production of an iron-storage protein called ferritin, which is part of an adaptive response to limit further oxidative damage. This cycle of damage and repair is a major driver of premature skin aging and, over time, increases the risk of skin cancer.
Fried and Overheated Foods
Cooking oils become a source of free radicals when they’re heated past their stability threshold. In the presence of heat, light, or trace metals from cookware, the fat molecules in oil lose hydrogen atoms from vulnerable points along their carbon chains. This creates lipid radicals, which then react with oxygen to form compounds called lipid hydroperoxides. These break down further into a cascade of additional reactive molecules.
Polyunsaturated fats (found in oils like soybean, sunflower, and corn oil) are especially prone to this because they have more of those vulnerable chemical bonds. Repeatedly reheating the same oil, as happens in deep fryers, accelerates the process significantly. When you eat oxidized fats, the free radicals and their breakdown products enter your digestive system and can contribute to oxidative stress in your body.
Alcohol and Liver Metabolism
Drinking alcohol sets off a chain of chemical reactions in the liver that generates free radicals at multiple steps. Your liver breaks down ethanol in two stages: first into acetaldehyde (a toxic, reactive molecule), then into acetate (which is relatively harmless). Both steps shift the balance of a key cellular molecule in a way that promotes radical formation.
Alcohol also ramps up the activity of a liver enzyme called CYP2E1, which processes ethanol but produces reactive oxygen species as a side effect. The more you drink, the more active this enzyme becomes, creating a self-reinforcing cycle. On top of that, the breakdown process generates a specific alcohol-derived radical called the 1-hydroxyethyl radical. Together, these pathways help explain why heavy or chronic drinking causes oxidative damage to liver cells even before full-blown liver disease develops.
Heavy Metals and the Fenton Reaction
Iron and copper, two metals your body needs in small amounts, can become dangerous catalysts of free radical production when they accumulate in excess. The key mechanism is called the Fenton reaction: dissolved iron reacts with hydrogen peroxide (a mild oxidant your cells produce routinely) and transforms it into the hydroxyl radical, one of the most destructive free radicals known. Hydroxyl radicals react with virtually any biological molecule they contact, and they do so almost instantly.
Copper drives a similar reaction. For decades, scientists assumed copper simply produced hydroxyl radicals the same way iron does, but more recent work shows the active oxidizing agent may be a copper-peroxide complex or a high-oxidation-state copper ion. Either way, the result is the same: rapid, indiscriminate damage to nearby molecules. This is why conditions involving iron or copper overload (like hemochromatosis or Wilson’s disease) cause widespread tissue damage over time.
Chronic Stress and Cortisol
Psychological stress doesn’t just feel harmful. It generates free radicals through a well-characterized hormonal pathway. When you’re under sustained stress, your brain signals the release of a cascade of hormones that ultimately drives your adrenal glands to pump out cortisol. Short bursts of cortisol are normal and useful. Chronic elevation is not.
Excessive cortisol directly contributes to the release of free radicals, and the resulting oxidative stress triggers inflammation, which in turn produces more reactive species. Nearly all forms of stress, whether physical, emotional, or psychological, activate both the hormonal stress axis and the sympathetic nervous system simultaneously. The combination drives a parallel release of cortisol and adrenaline-like compounds, both of which promote inflammation and oxidative damage. This shared mechanism is one reason chronic stress is now recognized as a contributing factor in conditions ranging from depression to neurodegenerative disease.
Exercise: A Dose-Dependent Source
Physical exercise increases free radical production, but the relationship is more nuanced than “more exercise, more damage.” Short, low-intensity activity (under a minute at about 30% of your maximum capacity) doesn’t appear to raise oxidative stress markers at all. Prolonged, high-intensity exercise clearly does, with measurable increases in markers of protein and fat oxidation in both blood and active muscles.
Here’s the twist: this follows a pattern biologists call hormesis, where a moderate dose of a stressor triggers beneficial adaptations, while too much causes harm. The free radicals generated during regular exercise actually signal your cells to build stronger antioxidant defenses, improve energy production, and repair damage more efficiently. Your body essentially trains itself to handle oxidative stress better. Recent research has even questioned whether intense exercise in healthy people ever truly reaches the level of radical production needed to cause net harm, noting that the evidence for extreme oxidant damage from vigorous training is not persuasive. For most people, the antioxidant adaptations from consistent exercise far outweigh the temporary increase in free radicals during a workout.