Acne is caused by a chain reaction inside your hair follicles: excess oil, dead skin cells that don’t shed properly, a shift in skin bacteria, and inflammation that turns a clogged pore into a red, swollen breakout. These four processes feed into each other, and the triggers that set them off range from hormones and genetics to diet, stress, and even air quality.
The Four Processes Behind Every Breakout
Every pimple starts the same way, as a “microcomedo,” a tiny blockage deep inside a pore that’s invisible to the naked eye. What happens next determines whether it stays small or becomes an inflamed cyst. Four overlapping processes drive that progression.
First, your oil glands ramp up production. The oily substance they make (called sebum) normally keeps skin moisturized, but when there’s too much of it, the excess pools inside the follicle and creates a breeding ground for bacteria. Second, the skin cells lining the inside of the pore become sticky and overproduce. Instead of shedding normally and washing away, they clump together into a plug that traps oil underneath. This plug is the physical blockage you’d recognize as a whitehead or blackhead.
Third, a specific skin bacterium called Cutibacterium acnes thrives in the oily, low-oxygen environment behind that plug. And fourth, your immune system reacts to the bacterial overgrowth by flooding the area with inflammatory signals, turning a simple clogged pore into a red, painful papule, pustule, or nodule. If the follicle wall ruptures under pressure, bacteria and oil spill into surrounding skin, and the inflammation spreads further.
How Hormones Drive Oil Production
Hormones are the single biggest reason acne tends to appear during puberty, flare before menstrual periods, and persist into adulthood for some people. Androgens (a group of hormones that includes testosterone) directly stimulate the oil glands to grow larger and produce more sebum. This is why acne often first appears in the early teen years, when circulating androgen levels climb sharply.
A related hormone called insulin-like growth factor 1 (IGF-1) amplifies the effect. IGF-1 causes oil-producing cells to multiply faster and ramp up their fat production. It activates a signaling pathway inside those cells that increases the enzymes responsible for making the lipids in sebum. The result is more oil, faster. When excess sebum fills a follicle, it shifts the local environment into a lipid-rich, oxygen-poor state that’s ideal for bacterial overgrowth.
There’s also a chemical link between excess oil and pore-clogging. One of the fats in sebum, squalene, can oxidize on the skin’s surface. The oxidized form directly triggers the cells lining the pore to overgrow and thicken, which is the second step in the acne chain. So hormonal oil overproduction doesn’t just fill pores; it chemically promotes the blockage that traps everything inside.
Why Skin Bacteria Matter More Than You’d Think
C. acnes lives on everyone’s skin. It’s a normal part of the skin’s ecosystem. So the question isn’t whether you have it, but which strains dominate and how diverse your skin’s bacterial community is. Research shows that acne development is tied not to the total number of bacteria, but to a loss of strain diversity. In lab studies, exposing skin to a mix of different C. acnes strains triggers far less inflammation than exposing it to a single strain alone.
One particular strain, known as phylotype IA1, is strongly linked to breakouts. On healthy facial skin, this strain makes up roughly 42% of the C. acnes population. In acne lesions on the face, it jumps to about 84%. On the back, the contrast is even starker: around 36% in healthy skin versus over 95% in acne lesions. When this strain dominates, it triggers the immune system to release a cascade of inflammatory molecules. Skin cells in and around the pore begin producing proteins that recruit more immune cells, amplifying redness, swelling, and tissue damage. The bacterium also releases tiny particles called extracellular vesicles that carry its inflammatory signals deeper into surrounding tissue.
Genetics Play a Major Role
If both your parents had acne, your chances of developing it are significantly higher. Twin studies consistently estimate that about 80% of acne susceptibility is heritable, making genetics the strongest single predictor of whether someone will get acne and how severe it will be.
A large genome-wide study published in Nature Communications identified 46 distinct locations in the human genome associated with acne risk. Some of these genes are involved in immune signaling and inflammation, others influence hair follicle shape and skin cell turnover, and some affect the growth factor pathways that control oil production. This helps explain why acne runs so strongly in families and why two people with identical diets and skin care routines can have completely different skin.
Diet: Sugar and Dairy Have the Strongest Links
Diet doesn’t cause acne on its own, but it can make existing acne worse by influencing hormones and inflammation. Two dietary patterns show up repeatedly in research: high-sugar foods and dairy products.
Foods that spike blood sugar quickly (sugary drinks, white bread, sweets) raise insulin levels, which in turn boost IGF-1 and androgen activity. That’s the same hormonal pathway that drives oil production. A systematic review of dietary studies found that people who consumed sugary drinks with 100 grams or more of sugar per day had roughly three times the odds of moderate-to-severe acne. Daily consumption of chocolate and sweets was associated with about 2.4 times the odds of acne compared to infrequent consumption.
Dairy, particularly milk, shows a consistent association across multiple large studies. Drinking more than three portions of milk per week has been linked to nearly 1.8 times the odds of moderate-to-severe acne. Skim milk appears slightly more strongly associated than whole milk in some studies, with odds roughly 1.4 to 1.9 times higher. The mechanism likely involves the natural hormones and growth factors present in cow’s milk, which can stimulate the same IGF-1 pathways in your body. Whey protein supplements, popular among gym-goers, show an even stronger link, with nearly four times the odds of acne in one large survey.
These are associations, not guarantees. Plenty of people drink milk and eat sugar without breaking out. But if you’re acne-prone, these foods can meaningfully worsen flares.
Stress and the Skin’s Own Hormone System
The connection between stress and acne isn’t just anecdotal. Your oil glands have their own independent hormone system that responds to stress signals. They contain receptors for corticotropin-releasing hormone (CRH), the same molecule your brain produces to kick off the body’s stress response. When CRH binds to receptors on oil gland cells, it stimulates them to produce more lipids and can also interact with testosterone and growth hormone to amplify the effect.
Studies examining skin biopsies found that CRH levels are dramatically higher in acne-affected skin compared to clear skin nearby or skin from people without acne. The stress hormone system in acne-prone follicles appears to be in overdrive, which helps explain why breakouts so reliably follow periods of emotional or physical stress. This system also activates local immune and inflammatory pathways, meaning stress doesn’t just increase oil; it directly promotes the redness and swelling of inflammatory acne.
Air Pollution and Skin Oxidation
Emerging evidence links air pollution to acne through a specific mechanism: the destruction of protective compounds on your skin’s surface. People living in highly polluted areas have significantly lower levels of squalene and vitamin E on their skin. In one study, residents of a heavily polluted district had squalene levels nearly half those of people in a cleaner area, and their vitamin E levels were more than ten times lower.
This matters because squalene is a natural component of sebum that helps protect skin. When pollution breaks it down through oxidation, the byproducts are themselves comedogenic, meaning they promote pore clogging. Lower vitamin E levels also reduce the skin’s ability to neutralize oxidative damage, creating a cycle of increased inflammation, more bacterial imbalance, and more clogged pores. For people already prone to acne, living or working in polluted environments can be a meaningful contributing factor.