This one’s tricky because a lot can be hidden. Essential plant oils’ environmental impact is based on two main factors: agriculture and chemical ingredients.
Agriculture: Essential oil has been used for thousands of years and is as Pagan as we are! But did you know it takes 60 roses to create a single drop of rose oil? Now you know why it’s so expensive! But to keep these costs down for consumers, the plants used are typically always conventional which means pesticides and herbicides and fertilizers were used on them, that can lead to algal bloom and declining bee populations. This also means they can be grown as a monoculture crop (versus polyculture, or diverse crops) which almost has to have insecticides to prevent total loss to bugs. And when it takes 63 pounds of melissa plant to create a single bottle of oil, that can be a lot of damage! Mountain Rose Herbs list several of their essential oils as USDA Organic but keep in mind, this only means that at least 70% of the ingredients are in fact, organic (which of course, is better than nothing). And some plants, like rosewood or sandalwood, are labeled vulnerable under the IUCN Red List of Threatened Species thanks to popularity and climate change.
Chemical Ingredients: Unfortunately, essential oil can be marketed as aromatherapy oil and this allows companies to hide the ingredients within (sometimes a hundred chemicals, sometimes 500!). Aromatherapy means it is now a cosmetic blend and so it does have to list everything under the International Nomenclature of Cosmetic Ingredients (INCI), but if there’s other chemicals not found in that ingredient list, it does not have to be known to you. This is because of a loophole created by the FDA in 1966 (Fair Packaging and Labeling Act) that allows companies to not have to list everything in their cosmetic products (including perfume and cologne) so as to legally protect proprietary blends or trade secrets. This law specifically extends to essential oils based on marketing terms from the manufacturer. Not only can this risk yours and your family’s health, but if this gets dumped down the drain, it risks river and marine health, too. And some of these cocktail blends make the essential oil an official hazardous material and if it’s old or rancid, needs to be deposited by your local hazardous-waste collection service and NOT simply thrown away. This includes the glass bottles that contain the residues- these can’t just be recycled! You can learn more about common blends, and how to dispose of them, on the MSDS.
Like I said, it’s tricky so always research the essential oil companies to be sure they vet their farming sources and contain as little added chemicals as possible and are listed.