The scavvy toxbomb is a grenade, but it works a little differently from the other standard grenades. Wherever the grenade lands, you place a permanent marker there. Anyone within 2" of that marker, unless protected by a respirator, is hit with a str4 hit on a 4+. This happens to anyone in that area when it lands, anyone within 2" at the start of their turn, and anyone that walks into that radius for the rest of the game.
Now, I see this weapon not so much as a grenade, but more an area denial weapon. This would be excellent to throw in doorways or chokepoints, to prevent the enemy from easily passing through those areas. But the standard grenade rules apply for these. You must target an enemy to throw these, just like every other grenade (except smoke grenades).
I think the requirement to have a target with this grenade really confines the use of it. Now, I know the reasons why you can't just throw grenades at any point you desire. Reasons such as, tossing into the middle of groups to hit the most enemies, making the grenade more powerful, and people cheesily saying "I can't target your model in hiding, so I'll just target 1 inch in front of him."
So what I'd propose is a different way of throwing it. You can either A: Use as normal, target an enemy and throw, or B: Throw it at a designated spot with no enemy within 5". This allows you to use it as a normal grenade, and allow you to use it as an area denial weapon which it seems to be, while preventing players from cheat hitting hiding models or models outside of the grenades range.
Thoughts?