Some chemicals, for example those used in cosmetics, jewellery and production processes, can cause contact allergies. These substances are called “skin sensitizers”. Toxicity testing is mandatory for new substances to ensure that as few people as possible experience allergic reactions to them. Mice and guinea pigs are currently used in this testing, but non-animal alternatives are being sought for these and other tests. This is not only important to reduce the number of animals used in experiments, but also to determine the safety of cosmetic ingredients, since they may not be tested on animals.

There is one animal-free method that uses human skin cells grown in a laboratory. This method came about through various tests that identified ten different genes that indicate with a high degree of certainty whether a substance can cause allergic reactions. However, this method cannot yet measure what quantity of the substance triggers those reactions.

In addition to this in vitro test on human skin cells, other tests have been developed that indicate whether a substance can lead to contact allergy. Combining information from different tests can help measure multiple biological processes that are important in studying how those allergic reactions occur. Van der Veen's doctoral research has shown that this method more accurately predicts whether a substance is a sensitizer or not. 

Considerable progress has been made over the past years in developing several new methods to identify skin sensitizers. But although the current animal-free strategy delivers valuable information, it cannot yet replace animal testing entirely. New methods must first be validated to further test their predictability and reproducibility. The expectations are, however, that it will not be long before an entirely animal-free test strategy can be used to identify skin sensitizers.