Because I know that many fungicides contain mercury, I was intrigued concerning your statement about people getting sick around the time defoliants are used on the cotton fields. I searched for a while and came across this interesting paragraph:

"Less well known is that mercury is a psychiatric toxin that because its neurotoxicity affects behavior. Mercury is a residue in gold and silver. Those who lust after gold and silver not only have been selectively self-poisoned, they have turned to the use of violence to accomplish their goals of acquisition. Historically this has entailed the the use of gunpowder. What do you suppose is involved in the manufacture of gunpowder? Mercury. DuPont family. Mercury is a necessary ingredient of plastics manufacture. The new car smell that so tantalizes wealth-seekers is....mercury. The gray fog on the inside of windshields of newer cars is....mercury. Mercury is a very cheap preservative used in products as diverse as vaccines and commercial fertilizer. Defoliant used prior to cotton harvest contains mercury. Read Q-tips, cotton swabs, as well as cotton clothing. We wear mercury in our mouths, in our jewelry, on our bodies, spray it on our heads, breathe it from our computers, TV sets, carpets....coal and other fuels....mercury."