Essential oils are often redistilled to make a small amount of oil go farther. They can make more profit that way. The problem is, when you redistill the oil you lose the therapeutic properties. In 1997 they produced 25 tons of lavender and 67 tons were shipped to the U.S. alone. These are the cream of the crop.
Great care is taken at every step of the process.
Chemicals are never used on the plants in the field. The time in the distiller is also important- cypress oil needs to be distilled for 24 hours. Many companies will distill at higher temperature for a shorter time. This will destroy the quality of the oil. The distiller must be no more than 50 miles away from the sight where the plants were harvested.