If you've ever developed an inexplicable rash after trying a new shampoo, makeup remover, or household cleaner, you might have encountered one of the most common contact allergens of the 21st century: isothiazolinone preservatives. Despite their tongue-twisting name, these chemicals have become ubiquitous in our daily lives – and they're causing allergic reactions at an alarming rate.
The Rise of a Modern Allergen
Isothiazolinones weren't always a problem. In fact, they were initially hailed as a breakthrough in preservation technology when they were introduced in the 1970s. These powerful antimicrobial agents were incredibly effective at preventing bacterial and fungal growth in products, and they seemed like the perfect solution for formulators looking to keep their products safe and stable.
However, what we've discovered over the past few decades is sobering. Contact allergy to isothiazolinones has reached what dermatologists now consider epidemic proportions. It finally appears to be downtrending (driven by changes in regulation of this preservative in Canada and Europe), but in my practice, I continue to see patients with isothiazolinone allergies very regularly.
By the Numbers: A Growing Epidemic
The statistics around isothiazolinone sensitivity are striking:
Contact allergy rates have increased by over 7-fold in the past two decades
Current studies show that 3-5% of the general population is now allergic to these preservatives
Among patients with suspected contact dermatitis, positive patch test rates exceed 10% in many studies
Children and young adults are increasingly affected, with some studies showing rates as high as 8% in pediatric populations
The Science Behind the Sensitivity
There are several types of isothiazolinone preservatives, but the main culprits are:
Methylisothiazolinone (MI) – Often used alone in rinse-off products
Methylchloroisothiazolinone (MCI) – Typically used in combination with MI
Benzisothiazolinone (BIT) – Common in household and industrial products
Once someone develops an allergy to isothiazolinones, even tiny amounts can cause significant reactions.
Where Are You Exposed? Common Sources
The challenge with isothiazolinone allergies is that these preservatives are everywhere. Here are the most common sources I see in my practice:
Personal Care Products
Shampoos and conditioners
Body washes and soaps
Makeup removers and cleansing wipes
Moisturizers and lotions
Sunscreens
Deodorants
Household Products
Laundry detergents (a major source of exposure)
Fabric softeners
Dishwashing liquids
All-purpose cleaners
Paint and coatings
Adhesives and glues
Unexpected Sources
Baby wipes and wet wipes
Hand sanitizers
Contact lens solutions
Printer inks
Leather products
Some medications and topical treatments
Disposable gloves
Recognizing the Reaction
Isothiazolinone allergies typically present as allergic contact dermatitis, which can look like:
Red, inflamed, itchy skin
Scaling or flaking
Small blisters or bumps
Skin that feels burning or stinging
The best way to confirm an allergy to isothiazolinone preservatives is through patch testing.