Lead Poisoning.

California has gone to lead-free ammo for good reason,
and the military is starting to move away from it.
Lead and lead compounds are very toxic, and tests of indoor
range instructors reveal dangerous lead levels in their blood.

Try to use low/no-lead ammo; at least TMJ (total metal jacket) ammo,
where even the base of the bullet has a metal jacket, particularly
for indoor ranges.
If you look at a slo-mo film of a firearm discharge, part of the
cloud is lead vapor, from the primer and the exposed base of a bullet.

Link to:"Lead in Teen Pits County Against Family"

There are many extreme-volume outdoor ranges around the U.S., one
at Camp Perry, Ohio. There are no bullet stops there, and Lake Erie is
the impact zone for most of the rifle lead, and has been for many decades.
This is just a small fraction of the lead dumped into watersheds nationwide.
http://www.gun-tests.com/performance/may97range.html

Though pure lead might be relatively stable in fresh water with low oxygen
and carbonates, multiply a small lead reaction by billions over decades,
and the effects could be serious, particularly for microscopic organisms,
bottom-feeding fish, clams, and other animals in the food chain.
There is also a slight galvanic reaction in "fresh: water when lead
is in contact with copper jackets.

I keep my indoor handgun range time to a minimum, and try to go at slow times,
even though they are ventilated now. You can sense the effectiveness of a
ventilation system by watching the movement of smoke.
I have enough lead exposure from our ugly history of leaded paint
and gasoline.

The current biological index (limit) for lead-exposed workers in the U.S.
is 30 µg/dL in blood specimens. Blood lead concentrations for indoor
shooting range instructors has been found to be 109 to 139 µg/dL.
[Disposition of Toxic Drugs and Chemicals in Man, 8th Ed., Biomedical Publications,
Foster City, CA, 2008, pg 823-826]

http://www.precisioncartridge.com/lead.html
http://www.dshs.state.tx.us/epitox/fact_sheets/firingranges_emp.pdf
http://www.rangeinfo.org/resource_library/facility_mngmnt/environment/Lead-OSHA.pdf
http://www.cdc.gov/niosh/topics/ranges/

http://www.cdc.gov/niosh/docs/wp-solutions/2010-113/pdfs/2010-113.pdf
http://en.wikipedia.org/wiki/Lead#Health_effects
.