Ask a RIFM Scientist: What is skin sensitization, and why study it?


Skin sensitization refers to a substance’s potential to cause an allergic skin reaction (such as red, bumpy, or itchy skin) in some people. Natural or synthetic ingredients may cause these reactions.

As part of its ongoing safety assessment of individual fragrance ingredients, the Research Institute for Fragrance Materials (RIFM) gauges the impact of these ingredients on the skin to determine their risk for causing skin sensitization.

Mihwa Na, PhD, leads RIFM’s skin sensitization team.

Q: Why is it important to study how fragrances affect the skin?

A: Skin is the largest organ of the body, and skin health is critical to our overall health and quality of life. Fragrances in personal care products, from soaps and shampoos to sunscreens and deodorants, come in direct contact with our skin.

Therefore, we must conduct safety evaluations on all fragrance ingredients and base our safety assessments of them on real-world exposure levels.

RIFM calculates consumer exposure to the ingredient using the Creme-RIFM Aggregate Exposure Model, which draws on measured data from surveys conducted by consumer habits experts Kantar. The data includes the amount of the ingredient in all products consumed and how people consume them, body weight and height, the specific body area where the ingredient is applied, and how long the ingredient will linger on the skin.

Q: Is skin sensitization a kind of irritation?

A: Skin irritation and skin sensitization are the results of different mechanisms in the body.

Irritation is a temporary surface response—such as itchiness or redness—that happens almost immediately when coming in contact with an irritating substance, such as very hot water. It goes away after the source of irritation is removed and may not return, depending on many different factors.

Sensitization, on the other hand, is an immune response that only develops in a fraction of people, and happens over time, after repeated exposures to a substance. Once a person’s immune system has been sensitized to a specific substance, they will always experience an allergic reaction to it, often at lower levels of exposure.

Q: How does RIFM identify potential skin sensitizers?

A: Historically, this was done using animal tests. But with the rise in ethical awareness and legislative bans on animal testing for cosmetics ingredients, RIFM pioneered the use of innovative scientific tools and methodologies that eliminate the need to test on animals without compromising on human health and safety.

The use of read-acrossusing an ingredient for which we have lots of data to help predict the toxicological profile of a structurally similar one for which little to no data exist—is one science-based alternative to animal testing.

Other strategies include in vitro testing (in test tubes or Petri dishes) and the dermal sensitization threshold, or DST. By using these strategies, RIFM has not needed to perform any skin sensitization tests on animals for over seven years.

Q: What do in vitro methods tell you?

A: For an ingredient to induce skin sensitization, it has to be able to penetrate through the skin and bind to skin proteins. The combination of the material and the skin protein then leads to a cascade of key events that eventually induce the immune response. A series of in vitro tests determine whether the ingredient is capable of causing each of these key events and, subsequently, skin sensitization.

Q: How does DST work?

A: DST is an exposure level below which there is a minimal risk for skin sensitization. Scientists identified the DST by studying data on known skin sensitizers. (The findings were published here and updated here.) If the exposure to a fragrance ingredient is below the DST, the ingredient may be considered safe at the current use level.

Q: What happens if an ingredient is determined to be a sensitizer?

A: Any fragrance ingredient identified as a sensitizer must be carefully managed. The RIFM safety assessment details maximum concentration levels for ingredients, based on the type of product in which the fragrance is used. The International Fragrance Association (IFRA) then sets and releases the Standards for new and existing formulations by which the Fragrance Industry is expected to comply.