In more dilute solutions, for example 1 : 40 000 or 1 : 20 000, of normal (Japan) camphor, which is an alicyclic ketone, tadpoles display a considerable degree of agitation. This agitated state persists for a very long period of time without the tadpoles exhibiting any further signs of abnormality. They can survive for days in solutions of 1:12000, without their movements being noticeably affected. In solutions of 1:5000 (1.3 x 10-3 mol/l), all spontaneous movements cease after about 15 minutes, although the tadpoles continue to respond to stimuli for about 1 hour. After complete narcosis has occurred, the tadpoles survive for about 6 hours, but their circulation becomes very slow. The tadpoles do not usually recover after being maintained in the solution for 8-10 hours.
Aqueous solutions of all essential oils have a rather marked narcotic effect, but many also have additional effects at the same time, e.g. paralysis of the heart. In very dilute solutions, they also frequently show a camphor-like agitating effect. These solutions also often affect the epithelia, which in many cases probably reflects the level of peroxide present. Although I have performed numerous experiments on the narcotic effects of these oils, mainly in the interest of certain problems in plant physiology, it seems unnecessary to explore these in further detail. It will suffice to say that narcosis usually occurs in solutions between 1:10000 and 1:100000 and that they are fatal within a few minutes to several hours, depending upon the essential oil used.
Studies of Narcosis Charles Ernest Overton Edited by Robert L. Lipnick United States Environmental Protection Agency Washington DC Chapman and Hall 1991 p.144