Brass is an alloy metal made with copper. With juices more toward pH 6 (acidic) the copper can leach out of the brass if the the juice makes contact with it, increasing the chance of inhaling vapor containing copper. This could be a problem as copper is toxic. However, many homes in the US have copper plumbing, so IMHO, I feel the actual risk is very slight, although contaminated e-juice would have a metallic copper taste to it. Yuck.