Now the question is whether his widow Natasha has the right to some form of compensation from me, and if so, on what basis?
Since you meant to keep this a simple example, I’m assuming that you think wrongful death would be a proper theory of recovery here.
This sounds like straight up, simple negligence to me, and thus all the questions that arise in negligence issues would arise here. Did you know about the downwind? Would a reasonable person have known? Was the death a reasonably foreseeable consequence of your action? What alternative means of disposing of the squirrels were available? Should you have known about them? Would you have been able to obtain them? And so on . . . (Any gripes you have with negligence as it stands would apply to all negligence issues, not just this one.)
The right violated is the same as it is elsewhere. Don’t touch my body, so to speak. I don’t think you have to “invent” any new right. Something came into contact with Boris’ body without his permission, and you made it happen. Your facts don’t suggest in any way that Boris expressly consented to your use of the chemicals. And I think implied consent in this case isn’t there either. You didn’t notify Boris or anything.
I think pollution presents interesting issues only because of proof problems. If you can’t show a direct harm, I say no recovery. If you can (like in Boomer v. Atlantic Cement IIRC), then recovery. Air is only the medium through which you commit the act. Whether it’s a bullet or toxic chemicals, the harmful object has to travel through something. Are you not responsible for a bullet that you fire because it has to travel through air to do its dirty work?