Let'em Breathe Space (version 2) - Lester del Rey

Eighteen men and two women in the closed world of a space ship for five months can only spell tension and trouble—but in this case, the atmosphere was literally poisoned. Who is trying to kill them and sabotage the mission? And why would they be doing this when they will die too? A deadly mystery unfolds inside the cramped space ship with fear growing daily as their air runs out.