Isothiazolinone Preservatives

A family of preservatives that became very popular in personal care and cleaning products around 2005, replacing parabens and formaldehyde releasers. The main one is methylisothiazolinone (MI). They work in tiny amounts and stop microbial growth in water-based products. Trouble is, they turned out to cause skin allergies at an unusually high rate.

Effective at very low concentrations, work well in water-based formulas, don't smell, and they were seen as a 'cleaner-feeling' alternative when parabens and formaldehyde releasers fell out of favour. Industry adoption was rapid.

Methylisothiazolinone (MI or MIT), methylchloroisothiazolinone (MCI or MCIT), benzisothiazolinone (BIT), Kathon CG (a common MI/MCI blend), and octylisothiazolinone (OIT). All belong to the same family.

Shampoo, conditioner, body wash, hand soap, dish soap, baby wipes (a major source historically), and water-based household paints. Less common now in cosmetics due to allergy concerns; still common in rinse-off products and cleaning.

Skin contact, almost entirely. Systemic absorption is minimal — the main issue isn't internal toxicity, it's skin reaction at the site of contact.

No strong evidence of pregnancy-specific harm. The main concern is skin sensitisation — developing an allergy that persists. Pregnancy doesn't change that risk much, but if you've been sensitised, pregnancy itchiness can be harder to manage.

No fertility concerns documented. Same skin allergy concern as for women.

Higher concern for skin sensitisation in children, especially via wet wipes used on the perianal and face areas. Once sensitised, an allergy to MI can last for life. Pediatric dermatology associations have specifically warned about MI in baby wipes.

Skin becomes more reactive with age. Already-sensitised adults may notice worsening reactions over time.

Very strong evidence for allergic contact dermatitis — MI was named 'Contact Allergen of the Year' by the American Contact Dermatitis Society in 2013, and contact-allergy clinics worldwide saw dramatic increases in MI sensitisation between 2010 and 2015. The EU restricted MI in leave-on cosmetics in 2016 and in rinse-off in 2017 based on this evidence. No strong evidence for systemic toxicity at typical exposures.

Low for most people who don't react. Real and significant for the estimated 1-3% of the population now sensitised — for them, daily contact triggers eczema, rash, and persistent skin issues.

Products preserved with phenoxyethanol, sodium benzoate, potassium sorbate, or pentylene glycol. Many major brands have already moved away from MI/MCI for leave-on products.

Easy. Read labels. Many products no longer contain it. Wet wipes and shampoo are the categories most worth checking.

If anyone in the house has unexplained skin irritation, eczema flares, or scalp itching, check the shampoo, body wash, baby wipes, and dish soap for 'methylisothiazolinone'. Try a switch and see if symptoms improve over a few weeks.

This is mostly a skin-allergy chemical, not a long-term hidden hazard. If no one in the household reacts to it, the systemic risk is low. If anyone has eczema or sensitive skin, it's one of the highest-yield things to check.

EU regulations 2016/1198 and 2017/1224 on MI restrictions, American Contact Dermatitis Society, and contact dermatitis literature. See References below.