Limonene is a monoterpene that occurs in citrus peel, and it has a strong generic citrus smell. Many terpenes are smelly, like carvone (spearmint or caraway). Interestingly, limonene is cheap and abundant enough to use as a semi-green degreaser or solvent.
Green or not, limonene isn’t totally safe. You can actually get sensitized to it – essentially developing an allergy – and then you’re in the unpleasant position of being allergic to citrus peels. Careful lab technique and a certain amount of luck have allowed me to avoid sensitization to any lab chemicals. And that’s a great thing – everyone I’ve talked to who was appreciably sensitized to a chemical said it seemed to turn up EVERYWHERE after that.
Please, share your sensitization stories below!