Ammonia can be very deadly too. I remember this event as if it were yesterday. (and now there are 18 wheeler wrecks daily in Houston) any thing can be a bomb, any chemical can be WMD....
http://www3.gendisasters.com/texas/18813/houston-tx-ammonia-truck-explosion-may-1976
The big push to jump to the alternative chlorine solutions came about from 9/11 emergency management planning to prevent terrorist strikes on these facilities. As a bonus to communities it also lowers your insurance risk and premiums.