20 miles from an ammo dump
SN, I live about 20 miles from a military base which trains, and stores chem weapons. The wind would usually blow anything away from us. Should we be worried?
On the first point you are right; the wind is your best ally.
Chemicals are a little out of my specialty. My forte is nukes. I will answer you with a hefty IMHO and address the question if I were in your shoes and compared to what I actually would do and am doing.
1/ A 72 hour kit is your best friend
2/ a half tank of gas, as a habit, is your next best friend.
3/ A 2-week supply of food, water and medicine, even before you break into your long-term food storage is a must.
Here is an article about building a safe room.
Homeland Security News » Safe Room
4/ Therefore, some cheap painter's tarps, duct tape and a homemade filter system complete your preparation. Download the free manual on Nuclear Survival. It is dated, but has some valuable parallel ideas. I sell the disk on eBay for $4. It has the complete manual and about 50 other e-versions of medical and survival manuals.
Masks will work for Military chemical agents, but what do you do to decontaminate your body. Some agents will kill you with just a small dot of agent on your skin. The Israeli's have child size masks, and they also shelter in place. But they are under threat of deliberate, delivered agents. Such was Dugway. A single canister breaking open and haphazardly dropping agent on the ground is a very minute threat.
A safe room is a much better idea.
In the Army, we would spend 30 minutes to 3 hours a day in full mask while on 30 day field exercises. Not fun. We also lived in our chemical suits 24 hours a day. Still, if hit with NBC weapons, we would still have taken casualties, both from the attack and the iffy process of decontamination. To have your children strap on a rubber mask that restricts breathing and makes you sweat like a pig is just cruel and impractical. For anyone who has bought a surplus mask in this thread, try wearing it for an hour and then tell me it is the best solution. I DARE YOU!
I read the wiki article on the Dugway incident.
Dugway sheep incident - Wikipedia, the free encyclopedia
It agrees that the threat is heavier than air particles dispersed deliberately as an aerosol. Wind, Temperature, humidity and luck all play into determining the dispersal pattern and the "hang-time".
Terrorists are stupid. Unprepared civilians are stupider.
NOTE: some duct tape does not stick well to plastic tarps. Check your stock out to make sure it works and rotate your duct tape thru normal use.
BLUF: I'm okay where you are and wouldn't worry. Anyone should have steps 1-4 on hand. Rest. Enjoy life. Look forward to retirement.