Acne can also result from mechanical or chemical skin irritation.

Mechanical skin irritation

Mechanical skin irritation refers to pressure, rubbing, friction, squeezing, and other forms of physical irritation. In medical terms it’s called acne mechanica. Examples include:

• Acne caused by a baseball cap or a backpack rubbing against the skin.
• Violinists sometimes get acne or other skin problems in the area where the instrument touches the neck.
• The friction between the head of an electric shaver and the skin can cause irritation and skin problems.
• Chin straps are another common source of mechanical skin irritation.
• Helmets and face guards.
• Bra straps can irritate the skin in the back.
• A few case studies show that resting your head on your hands, for example while reading a book, can also trigger acne mechanica.

Spa and facial procedures that combine skincare products or devices with massage and rubbing of the skin can also trigger irritant-type acne.

A combined use of topical products and skin care associated with devices/vigorous massages may cause inflammatory cutaneous lesions as well.

Dreno, B., Bettoli, V., Perez, M., Bouloc, A. & Ochsendorf, F. Cutaneous lesions caused by mechanical injury. Eur J Dermatol 25, 114–21 (2015). https://www.ncbi.nlm.nih.gov/pubmed/26069089

Treatment

• By far the most important thing is to stop the irritation The problem most likely persists, at least in some degree, as long as the irritation remains.
• Try using salicylic acid or alpha-hydroxy acids (AHA) to peel the skin. Mechanical irritation often leads to thickening of the skin and increased keratin production, which leads to clogged pores. Salicylic acid and AHAs can peel the skin and keep the pores open.
• Consider using topical retinol. Retinol is a gentler form of prescription tretinoids. It can reduce keratin production and helps to keep the pores open.
• Use tea tree oil or other antibacterial substances. Mechanical irritation can change the type of oils the skin produces and thus lead to changes in the bacteria inhabiting the skin, which can cause inflammation in the affected area. Tea tree oil helps to control a large variety of bacteria and yeasts that live on the skin.

Chemical skin irritation

Various chemicals are another source of skin irritation. In medical speak this is called acne cosmetica.

Types of chemical irritation include:

• Overuse of harsh products, such as prescription tretinoids or benzoyl peroxide
• Allergic reaction – I once got an email from someone who got acne from her dog; she discovered that she was allergic to animal fur and the dog’s fur irritated her skin and caused acne.
• Products that contain high doses of comedogenic ingredients

Overuse of harsh products

Overuse of harsh acne treatment products is the most common source of irritant-type acne. While undoubtedly effective, many established acne products cause collateral damage to healthy skin cells and often end up irritating the skin – especially when overused.

Let’s take benzoyl peroxide as an example. Studies have shown that a single application of 10% BP depletes 93% of the skin’s vitamin E reserves.

Earlier we discussed the role sebum oxidation plays in the acne cycle, and the role antioxidants play in preventing it. Vitamin E is one of the most important antioxidants in the skin. Being fat-soluble, it can protect the fatty acids in sebum from oxidative damage better than vitamin C and other water-soluble antioxidants.

BP can also destroy the skin barrier function and increase water loss through the skin. Excessive water loss makes the skin dry and sensitive.

Depletion of antioxidant defenses and damage to the skin barrier function leave the skin more vulnerable to acne.

And when acne gets worse, people often make the mistake of heaping on even more irritating products. You can see how this leads to a situation where the skin is sensitive, irritated, and always breaking out.

Hallmarks of chemically irritated skin include:

• Sensitivity
• Redness
• Dryness
• Itching in some cases

Treatment

The solution is obvious but painful. You have to stop using irritating products and allow your skin to recover. Alas, initially this can make your face explode with acne; now the skin is both damaged, and you lose whatever power the products had in keeping acne in check.

Fortunately, there are ways to make this transition period easier. Here’s what I would do if I were in that situation:

1. Immediately stop using all forms of skin peels, scrubs, and exfoliants. These are among the most damaging procedures for the skin.
2. Use topical antioxidants to replenish the depleted antioxidant reserves. Good candidates include vitamins C, B3 and E, as well as green tea.
3. Use a good moisturizer or use oils (not coconut oil) to boost the skin barrier and reduce moisture loss. Preferably, look for something that contains linoleic acid (LA). LA is crucial for the health of the skin barrier, and several studies have shown acne patients have much less LA in the skin than people without acne. LA can reduce runaway inflammation in the skin by reducing the number of free radicals white blood cells release.
4. Gradually wean yourself off the irritating products. Start by slowly reducing the frequency of use. Later on, you can switch to less irritating products. For example, you can swap tretinoids to benzoyl peroxide and then benzoyl peroxide to salicylic acid.

I would do the steps 1 to 3 for 2 to 4 weeks before moving on to step 4. Over time, your skin will crawl back to health and become less irritated and sensitive. And more than likely, you’ll also get less acne.

Allergic or irritant reaction

Certain substances can cause allergic or irritant reactions when they come in contact with the skin. In most cases they cause a red and itchy rash. And while these aren’t usually mistaken as acne, it’s possible they cause skin damage that leaves the skin vulnerable to breakouts.

Symptoms include:

• Itching in most cases
• Burning
• Stinging
• Pain
• Redness

Unfortunately, the list of chemicals known to cause irritant and allergic reactions is long, ever-changing, and perpetually incomplete. If you suspect something in your skin or personal care products is causing allergic or irritant reactions, the best thing to do is to visit a dermatologist and take all the products you use with you. The doctor can do a patch test to check what your skin reacts to.

For DIY investigations, I would start by looking at the following:

• Shampoos, soaps, and cleansers – Surfactants (such as sodium lauryl sulfate) are a common cause of skin irritation.
• Sunscreens – The damage caused by UV rays may sensitize the skin to chemicals in the sunscreen.
• Preservatives – Such as formaldehyde, formaldehyde releasers (quaternium 15, diazolidinyl urea, dimethyloldimethyl hydantoin, imidazolidinyl urea), iodopropynyl butylcarbamate, and methyldibromoglutaronitrile. Ironically, the much-maligned parabens continue to be among the safest and least irritating preservatives.
• Fragrances.
• Essential oils – Especially those from lemon and other citrus fruits.
• Topical medications.
• Hair dyes.

Unfortunately, you can’t always rely on labels to accurately state what the product contains. For example, a 2015 study analyzed 245 skin care products patients suspected cause skin irritation. Our of these, 58 contained formaldehyde or formaldehyde releasers, and 26 of the 58 products were not declared to contain formaldehyde or formaldehyde releasers.

This is one more reason to work with a doctor familiar with the subject.

Comedogenic ingredients

Comedogenicity attempts to quantify the degree to which ingredients in skincare products clog pores and cause acne. Ingredients are rated on a scale of 0 to 5, with 5 being the most comedogenic and liable to cause acne.

While these ratings sound useful in theory, in practice they aren’t that useful. The problem is that the tests used to generate comedogenicity data don’t reflect real world conditions, and as a result, many of the ingredients rated as comedogenic don’t cause acne when used in skincare products. I explained this in more detail in the comedogenicity page of the topical treatments section.

That being said, comedogenic ingredients can be a problem for some people. If you have a reason to believe your skin care products cause problems, do check the label for comedogenic ingredients.

The internet is full of exactly the wrong kind of advice on how to check your skin care products for comedogenic ingredients. Here are a few tips to do it properly:

• Pay attention only to the ingredients rated as highly comedogenic (rated 4 or 5). Anything less than that and you don’t have to worry about it.
• Only check the first five ingredients in a product. The ingredients are listed in the order of descending concentration; meaning that the ingredient with the highest concentration is listed first, the second highest concentration is listed second and so on. Once you get past the first 3 to 5 ingredients, the concentrations become too diluted to cause problems.