You shouldn't worry about chromium 6. The greater concern would be taking great big puffs deep into your lungs of that warm, moist bacteria you're creating in that exceptional bubbling aquatic bacterial environment (think petri dish on steroids,) called a bong.

Or, maybe you put Clorox in your bong, then the dishwasher after every use.

Learn to roll'em. It's the healthiest way to puff.