Certain essential oils contain allergens that cause allergic contact dermatitis (eczema) when in contact with skin. Tea tree oil, Lemongrass oil and Sandalwood oil are some of the essential oils that contain allergens. 

The Cosmetics Regulation 1223/2009 lists the 26 most-known allergenic substances. If you have sensitivities it is best to avoid soaps and natural skincare with essential oils and fragrances.

By law, if a product still contains essential oils or fragrances, specific dermal limits must be adhered to. In addition, these allergens must appear on the label of your cosmetic product.

So keep an eye out for allergens such as Lyral, Citral, Citronellol, Eugenol, Limonene that should be added towards the end of your ingredient list. For a full list check out: https://www.ecomundo.eu/en/blog/cosmetics-allergens-europe-compliance