Steam distillation is one of several methods used in the production of essential oils. Hydrosols or aromatic waters are co-products of this process. When plant material is put into a still, distilled water is added and heat is introduced creating steam. This steam causes the cell walls of the plant to soften and burst open releasing all of the active components found within the cell. Once the steam cools down and again and becomes liquid the essential oil separates from the hydrosol. Most essential oils are lighter than water and they will float to the surface. The essential oil is then carefully removed leaving behind the very aromatic water or hydrosol.
Hydrosols are much less concentrated than essential oils however they do contain tiny droplets of essential oil. In addition, all the water soluble components from the plant are captured in the hydrosol. These components are not present in the essential oil. Hydrosols can be thought of as a more complete extract of the original plant whereas essential oils are a very concentrated portion of the plant. Both have their uses and benefits in aromatherapy.
Unfortunately, there has not been much scientific study on use of hydrosols. Regardless, many aromatherapists use them very successfully in their practice. We are slowly gaining more evidence of their therapeutic value as more aromatherapists document and share their own observations. Hydrosols are very safe topically. They can also be used safely internally when they are very fresh and very pure. It's important to know the process of the distillers who are making the hydrosols that you are using. You can also make your own hydrosol at home using a few simple steps if you're feeling adventurous.
The shelf life of hydrosols varies between plant species and distillation techniques. Most well made hydrosols keep for several months and even years. Once bacteria is introduced, hydrosols, like anything, can be harmful to ingest so it's important to work with an experienced registered aromatherapist before using any internally.