Long story short: When a plant part is processed through steam distillation, the essential oil separates from the water. That water is the Hydrosol! Hydrosols still have therapeutic benefits, but are safe and gentle for kids under 2 yrs old.
For example, I use Helichrysum Hydrosol for dry eczema skin. If I were to make a body wash for my skin during a cold winter, I would combine 4oz water, 4oz Helichrysum hydrosol, 3 oz Dr. Bronner’s sensitive castile soap, a squirt of Tamanu carrier oil, and 20 drops of Lavender essential oil. The Helichrysum hydrosol would keep my skin wonderfully moisturized, and the Tamanu oil would promote healthy skin healing and growth.
Learn more at: http://www.naha.org/explore-aromatherapy/about-aromatherapy/what-are-hydrosols