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.