I thought that when the ISA would go to invade on Helgan, THEY should be wearing the masks.
I mean, if the Helghast initially survived by using the helmets and masks before adapting (by mutation possibly, so I guess it could be viewed as evolution), then how come an invading military force doesn't need any of that. You know, the War of the Worlds theme could easily be applied to the ISA invading the alien (to them) world filled with unknown hazards.
When the Helgast invaded Vectra, they had breathing aid. It seams logical that when the Vectrans would invade Helgan, they should have the breathing aid.

More than fifty years later, veteran soldiers are only now taking class action against the British government for health issues that arose much later in their life due to exposure to nuclear testings...

Sucks to be a grunt doesn't it? :p

Yes but I was thinking more in the line of reducing the probable physical handicap the soldiers would suffer in battle. Not enough oxygen, or "poisons" in the air that reduce oxygen absorption and you have one tired army.

[OffT: Does the Posts counter reset after a while?]

according to the backstory on the website.. the mask where only needed by the lower class.. working in the mines and the refinerys.. .and the effect took years to show.. so isa eithe doesent know about this.. or they think that they will be off the rock before its getting dangerous.... the only reason all hellghast now wear the masks is because once scolar visary came to power he introduced the mask.. to all classes.. makiing them all equal.. and the mask the symbol of their evolution from man to .something else .. .they dont need them anymore.. but they wear them as a symbol of being a hellghast

Aha, thanx. So it's not considered imediately dangerous to attack Helgan, considering that most of the fighting would take place on the relatively non toxic (compared to the mines and refineries) surface of the planet I think.

More on topic: when I first was Visari in Killzone 1, I imediately thought of Mussolini. He's speaches are more Mussolini like than Hitler like don't you think?

maybe its a little of both.. but i remember it reminded me of a speech hitler gave in.. i think it was 1938... the part about "10 years ago i asked for time...... in that time i have rebuild...etc" that was pretty much directly from hitlers speech.. only that it was 5 and not 10 years   what he all did in those 5 years can be read in detail somewhere else. .but among other things he illegally rearmed the military  ..reintroduced the.. i think you call it drafting system ...for mandatory military service for all citizens of age 17+  .rebuild the airforce.. the navy... .and developed a lot of new weapons ,which was all forbidden by the versailes treaty from 1918

and when it comes to the invasion of vecta... there are also similarites to speeches he gave when the war started.. i just cant find that particular speech.. but ill also be looking into a few more mussolini speeches

Talking about the nature of Helghan, it's cited in the opening CG to the game. While Visari is giving his speech, one of the news topics going at the bottom of the screen is,

"Helghan's atmosphere: a threat to our troops?"

If we don't see any affects of it in Killzone 2's storyline, then it's probably because it's something that slowly affects those that unprotected.