Oxidative stress and the skin — what it is and why it matters.

The term "antioxidant" appears in roughly a quarter of all skincare product descriptions. Free radicals are invoked as a ubiquitous threat. Oxidative stress is cited as a driver of ageing, pigmentation, and inflammation. The claims are accurate — oxidative stress does all of these things — but the specifics are usually lost in the translation from biology to marketing.

What oxidative stress is

Every cell in the body continuously produces energy through metabolic processes. A byproduct of this production is reactive oxygen species (ROS) — molecules with an unpaired electron that makes them chemically unstable. These molecules are sometimes called free radicals. The instability causes them to react with, and damage, nearby cellular structures: proteins, lipids, and DNA.

In small amounts, ROS are a normal part of cellular function — they are produced as part of immune responses and act as signalling molecules. The problem arises when their production outpaces the body's capacity to neutralise them. That imbalance is oxidative stress.

What triggers it in the skin

The skin is the body's outermost layer and is continuously exposed to the most significant environmental sources of ROS.

UV radiation is the primary trigger. UV photons interact with chromophores in the skin — melanin, DNA, proteins — generating singlet oxygen and other ROS directly. This is the mechanism behind UV-induced collagen degradation, hyperpigmentation, and mutagenic DNA damage.

Ambient air pollution — particulate matter (PM2.5), ozone, nitrogen dioxide — generates ROS on contact with skin. Epidemiological studies have associated chronic urban pollution exposure with accelerated skin ageing, independent of UV.

Smoking delivers a sustained dose of ROS and depletes cutaneous antioxidant reserves. The effect on collagen is measurable and dose-dependent.

Infrared radiation and heat are less well-studied than UV, but increasing evidence links infrared exposure to ROS generation and collagen-degrading metalloproteinase activation.

What the damage looks like

Oxidative damage in the skin is cumulative. ROS attack collagen and elastin fibres directly, disrupting the structural proteins that maintain skin density and elasticity. They activate matrix metalloproteinases — enzymes that degrade the extracellular matrix. They trigger melanocyte hyperactivation, contributing to uneven pigmentation. They damage DNA, which over decades accumulates as photocarcinogenesis risk.

The visible result of chronic oxidative stress — fine lines, loss of firmness, uneven tone, dullness — is so closely associated with UV exposure that the two are often conflated. But they are related rather than identical: UV is the largest single source of cutaneous ROS, but not the only one.

The endogenous antioxidant system

The body maintains an endogenous antioxidant defence — enzymatic systems (superoxide dismutase, catalase, glutathione peroxidase) and non-enzymatic compounds (glutathione, uric acid, coenzyme Q10) that neutralise ROS before they cause damage. Vitamin C and vitamin E are the primary exogenous antioxidants the body concentrates in the skin.

Chronic UV exposure depletes these reserves faster than diet can replenish them. That is the biological rationale for topical antioxidant application.

Topical antioxidants

Applied topically, antioxidants intercept ROS before they reach cellular targets. The most evidence-backed options:

Vitamin C (L-ascorbic acid) is the most studied topical antioxidant. Water-soluble, it scavenges ROS in the aqueous compartment of the skin and regenerates vitamin E. Effective at 10–20%, pH below 3.5, in stable opaque packaging.

Vitamin E (tocopherol) is fat-soluble, protecting lipid membranes from peroxidation. It works synergistically with vitamin C — when vitamin E donates its electron to a free radical and becomes oxidised, vitamin C regenerates it.

Ferulic acid stabilises the vitamin C and E combination and doubles the photoprotective effect of the pair. The combination is supported by peer-reviewed research demonstrating measurable photo-oxidative protection.

Niacinamide is primarily an anti-inflammatory and barrier-strengthening ingredient, but it also suppresses oxidative-stress-triggered melanocyte activity — relevant to the pigmentation arm of oxidative damage.

Green tea (EGCG) has antioxidant activity and mild anti-inflammatory effects at concentrations found in well-formulated products.

The practical implication

Topical antioxidants are morning ingredients. Their role is preventative — intercepting environmental ROS before they cause damage. They do not reverse existing damage; they reduce the rate at which new damage accumulates. Applied under SPF, they provide a layer of protection that SPF alone does not offer: SPF absorbs or reflects UV photons; antioxidants neutralise the ROS that UV still generates even with SPF applied.

The sequence: antioxidant serum, then SPF. Both, consistently, every morning.

Night-time application of antioxidants is not harmful — some repair mechanisms occurring during sleep are sensitive to oxidative stress — but the primary rationale for antioxidant use is morning environmental protection. Night is when barrier-repairing and cell-turnover-promoting actives are most relevant. The two categories are complementary: protection in the morning, repair at night.