Heavy gauge plastic sheeting and duct tape will be your friend. Basically think of air flow in your home... windows, doors, chimney, heating vents, attic access, etc. You could last short term with complete sealing but O2 would run out in a short time. Evacuation would be best bet. As well it would depend on what the chemical agent is.

I lived around the corner of Plastimet at the time of the fire. For years after the fire we had to get our soil tested for toxins from the burned plastics. Just because the product isn't listed as being harmful as is, doesn't mean that it wouldn't be if it was burned. Besides the toxins, our neighbourhood was over run with mice for months after. That carries a whole set of other toxic issues. My best advise would be to leave if asked.

In such a case I think bugging out would be in order. Look to what you have already that could be used as mobile shelter...if you have a pickup truck, consider one of those camper units that goes on the back...used of course, or even just the regular bed cap. A tent would suffice in warm weather, but you have to find a place to pitch it. An attached camper can be used in any parking lot or roadside rest area. Have a bug out bag ready with food, water and clothing for 3 days or more. Extra survival gear can be kept in the camper or truck.

I'm of the opinion that bugging out would also be a really good idea. For the obvious reasons, but I'd be super paranoid about the harmful stuff floating in the air you couldn't see/smell if that place had an accident.

I think you mentioned you had some land there as well? If your growing food on it, I think it would be really smart to have the soil tested at yearly intervals. I worked with a fellow for a few years that lived near Inco in Port Colbourne and despite his being told a couple of times by the company and the Ministry of Environment that there was no risk, he had an independent test done on his yard soil and it contained contaminates - like nickel and cobalt, some others I don't remember - from the plant's emissions. The officials told him that nothing was wrong, but he was given a sum of money in compensation for it. His wife also died of cancer (they had lived there for 27 years and it wasn't conclusively linked to this, just strongly suspected) and his oldest girl was sick (something the doctor couldn't really pin down) while I worked with him.

Not to make you overly alarmed, just wanting to point out that if you live near industrial plants take care of yourself while you are there.

Runs With Scissors

I am a great believer in that unless your home or apartment is compromised by fire, flood or contamination you are immeasurably better off staying where you are. A chemical disaster fits the bill for bugging out. In many cases the authorities first response might be to shelter in place. Closing all doors, windows etc and turning off air conditioning and anything that draws external air. For a very first response that is a sound approach. Evacuation would come later. Although this is a SHTF situation is not usually very widespread. Therefore authorities will have evacuation centres to go to. Do you have at least your 72 hour BOB ready by the door? Personally I would head out of town and book into a nice hotel but if you want to save money what a wonderful opportunity to pay a visit to those out of town relatives or friends that you have not seen in a dogs age.