They can irritate mucous membranes, cause dermal irritation and may cause sensitization in some individuals. Use in moderation, in low dilution. They are best avoided during pregnancy. White thyme is not a 'complete' oil and is often adulterated. Lemon Thyme and 'linalol' types are in general less toxic, non-irritant with less possibility of sensitization, safe for use on the skin and with children.
Aromatherapy is a complementary therapy.
This information should not be treated as a substitute for the medical advice of your own doctor or any other health care professional.