Breakouts happen when oil, dead skin cells, and bacteria clog your pores and trigger inflammation. That’s the short answer, but the real question most people have is why it keeps happening, and why certain things seem to make it worse. The causes range from hormones and genetics to diet, stress, and even the air you breathe.
The Four Things Happening Inside a Breakout
Every pimple forms through the same basic chain of events. First, your skin produces too much oil (sebum). Second, dead skin cells don’t shed properly and instead pile up inside the pore, forming a plug. Third, a specific type of bacteria that naturally lives on your skin multiplies inside that clogged pore. Fourth, your immune system responds to the bacterial overgrowth with inflammation, producing the redness, swelling, and sometimes pain you see on the surface.
All four of these steps matter. A blackhead is what happens when the pore clogs but inflammation stays minimal. A red, swollen pimple or a deep cyst means the inflammatory response has kicked into high gear. Understanding this helps explain why so many different triggers can cause breakouts: anything that increases oil production, slows skin cell turnover, feeds bacteria, or ramps up inflammation can set the process in motion.
Hormones Are the Biggest Driver
Hormones, particularly androgens like testosterone, are the primary reason your skin overproduces oil. Androgens bind to receptors inside the oil glands and switch on genes that ramp up sebum output. This is why acne peaks during puberty, when androgen levels surge, and why many women break out around their period, during pregnancy, or when starting or stopping hormonal birth control.
It’s not just about how much testosterone you have in your bloodstream. Your skin converts testosterone into a more potent form called DHT right inside the oil gland itself. Some people’s skin is simply more efficient at this conversion or more sensitive to the hormones that arrive there, which is one reason two people with identical hormone levels can have very different skin.
Genetics Account for About 80% of the Risk
If your parents had acne, your chances are significantly higher. A twin study published in the Journal of Investigative Dermatology found that 81% of the variation in acne severity was attributable to genetics. Only 19% came from environmental factors unique to each individual. Your genes influence how large your oil glands are, how your immune system responds to clogged pores, and how quickly your skin cells turn over. None of that is something you can control, but knowing it helps explain why some people break out despite doing “everything right.”
Stress Makes Existing Acne Worse
Stress doesn’t just feel like it causes breakouts. There’s a direct biological pathway. When you’re under chronic psychological stress, your adrenal glands pump out extra androgens, which increase oil production. At the same time, your nervous system releases signaling molecules called neuropeptides that trigger inflammation throughout the body, including in the skin.
One of these neuropeptides activates mast cells in the skin, causing them to release a cascade of inflammatory chemicals. The result is that pores that might have stayed as minor clogs become red, inflamed pimples. This is why students often break out during exams, or why a stressful month at work can coincide with a flare. The oil glands in your skin actually function as a semi-independent hormonal organ, responding directly to stress signals from the brain.
High-Sugar Diets Have a Real Effect
The connection between diet and acne was dismissed for decades, but recent systematic reviews confirm that high-glycemic diets, those heavy in sugar, white bread, and processed carbohydrates, have a modest but significant effect on breakouts. Foods that spike your blood sugar also spike insulin, which in turn increases oil production and skin cell growth inside pores.
The numbers are striking in clinical trials. People placed on a low-glycemic diet (more whole grains, vegetables, and protein, fewer refined carbs) saw their total acne lesions drop by about 59%, compared to 38% in control groups eating normally. Inflammatory lesions, the red painful ones, dropped even more dramatically. In one study, the low-glycemic group saw a nearly 71% improvement from baseline. Frequent sugar intake raised the odds of acne by about 30%, and drinking more than 100 grams of sugar from soft drinks per day tripled the odds of moderate-to-severe acne.
This doesn’t mean sugar “causes” acne on its own. But if you’re already prone to breakouts, a diet heavy in refined carbs is pouring fuel on the fire.
Dairy and Breakouts
Milk and dairy products contain proteins that raise levels of insulin-like growth factor 1 (IGF-1) in the blood. IGF-1 stimulates oil production and promotes the kind of cell growth inside pores that leads to clogs. Dairy also naturally contains hormones, including androgens, that can push oil glands into overdrive. Meta-analyses of observational studies consistently find an association between dairy intake and acne, though the effect appears stronger for skim milk than for full-fat milk or cheese. The exact reason for that difference isn’t fully settled, but one theory is that processing concentrates certain hormonal compounds.
Not All Skin Bacteria Are the Problem
The bacterium most associated with acne lives on everyone’s skin, whether or not they break out. What matters is which strains dominate. Certain subtypes are highly inflammatory: they release particles that activate your immune system and trigger the production of chemicals like IL-8 and TNF-alpha, which cause redness and swelling. Other strains of the same species are actually protective. They appear to reduce inflammation and may even help control oiliness. This is why aggressively sterilizing your skin with harsh cleansers can backfire. You may wipe out the helpful strains and leave room for the inflammatory ones to take over.
Friction, Heat, and Pressure
Physical irritation causes a specific type of breakout sometimes called acne mechanica. It shows up as small red bumps and pimples in areas where something rubs, presses, or traps heat against your skin. Common culprits include tight-fitting hats, helmet straps, backpack straps, headbands, sports pads, bra straps, and even resting your chin in your hand. Athletes are especially prone to this because of equipment and sweat. The combination of friction, pressure, and trapped moisture creates the perfect environment for pores to clog.
If your breakouts follow a pattern that matches where clothing or gear touches your skin, this is likely a factor. Wearing moisture-wicking fabrics, loosening straps where possible, and showering soon after sweating can make a noticeable difference.
Air Pollution and Your Skin Barrier
Emerging evidence links air pollution to acne flares. Particulate matter and nitrogen dioxide appear to damage the skin’s protective barrier, generate oxidative stress, and disrupt the balance of bacteria on your skin. Pollutants also deplete antioxidants like vitamin E from the skin’s surface, making it more vulnerable to inflammation. Studies have found that people in areas with higher pollution levels tend to have increased oil production and more comedogenic (pore-clogging) changes in their skin’s chemistry. If you live in a high-pollution area, cleansing at the end of the day to remove particulate buildup and using products with antioxidants may help offset some of this effect.
Why Breakouts Cluster in Certain Areas
Where you break out often hints at the cause. The forehead and nose (the T-zone) have the highest concentration of oil glands, so they’re most responsive to hormonal shifts. Jawline and chin breakouts in women frequently track with menstrual cycle hormones. Cheek breakouts can stem from phone screens, pillowcases, or touching your face. Breakouts along the hairline may come from hair products (pomades, oils, heavy conditioners) migrating onto the skin. And breakouts on the chest, back, or shoulders often involve friction from clothing or gear combined with sweat.
Most people don’t have a single cause. Breakouts typically result from several of these factors layering on top of each other: a genetic tendency toward oily skin, a stressful week, a diet heavier than usual in sugar, and a pillowcase that hasn’t been changed in two weeks. Addressing any one of these won’t eliminate acne entirely if other factors remain, but reducing several at once is often what makes the visible difference.