Reply to steve's Post: I also neglected to qualify what I meant by ppm. I meant ppm by volume. So, to get moles of any ideal gas, we divide by the number of liters of air there are in any a mole of anything, which is 24.45 (@STP?). So benzene moles/L in air is equal to benzene mol/L in water. Urine is mostly water, but not all water. Urine contains lots of ions and creatine, and protein, and other stuff. So one volume of urine is not going to be exactly the same as another. However, deterministically,rather probablistically, we might take a mean urine density for a healthy, 70 Kg adult of 30 years, whose fluid intake is exactly 2L of water of day. However, I don't what such a value might be, nor have found any sources. So, when I saw your website I thught, hey, these guys think they can answer anything, so.....But, you, in fact did answer the question, as posed, in a correct way, because I didn't specify that I was not interested in benzene by weight. So, it is only fair that that I pay you.