According to MiTS,

It gains that benefit any time it's in a toxic domain, as they're not bound to any one domain unlike other nature spirits. The rules are simply saying that they drop in power when they're forced to enter a "clean" area while in service to their master.

In your examples, it would be Force 7, Force 7, and Force 6. It would only be Force 4 if in a non-toxic domain.

I would suggest the effective Force increase would only be in the domain they were summoned in, although it is not unreasonable that a spirit birthed in the sterile hearth of a megacorporate condo could just as easily flourish under the alternate despair and hostility of a delapidated apartment building in the ghetto.

So, I suggest restricting the Force increase to the domain it was summoned in, but you may at your whim decide to allow it to gain power in similiar toxic domains. Imagine a Toxic Spirit turf war over a particularly vile piece of real estate...oh, wait, that would be the Yucatan.

My take, based on the line "facing toxic spirits in their home domain is a remarkably bad idea," is that the spirit only gains the increase from being in the domain in which it was summoned.

I was hoping for a bit more healthy debate, but meh.

"Facing toxic spirits in their home domain is a remarkably bad idea" means exactly that. In no way does that means it's the only place that they gain the bonus, just that they're guaranteed to wherever they were summoned since they can only be summoned in a toxic domain.

If it was intended to work the way you're implying, the rules would read more like "...in their home domain, a toxic spirit gains..." but it doesn't; it says in a toxic domain, they gain the bonus, after clearly stating that toxic spirits have the ability to travel from domain to domain as one of their unique characteristics.

Well, toxic spirits of man shouldnt gain the bonus in toxic nature domains, and vice versa, because toxic nature domains and toxic human domains occur for different reasons.

I would consider separate toxic domains into various types, of the top of my head there would be psychological, poison and destruction. These could occur in any traditional domain be it a domain of nature or man (psychological would be rare in nature and destruction rare in man but they would be possible) a toxic spirit would only gain bonuses in a aria that was toxic for the same reason as where it was summoned, toxic magicians would have similar limits.

This restriction however is not what the books impose, just something I was toying with.

Edward

OK, I am starting to lean toward the Funkmeister's view on this, but I like Edward's concept as well. I may house rule it that way in the future.

I am currently running a free spirit intense game, and its about to get interesting (if the PCs don't ruin everything with good planning).

Anyone else want to confuse me more?