I have seen some questions asking whether some atmospheric compositions are survivable for humans. Most of them focus on long term survival, as in people being able to breathe indefinitely without the need for special apparatus, or at least with a breather.
I want a variation of the second case. I am writing a story that happens in a planet where people can breathe without any equipment for about a day, perhaps more, but this will ultimately kill them. So they use breathing gear when exploring the place, and they have no cause to panic in case the equipment breaks down as they can just go for a spare... But if someone gets lost and can't find a new breather, they know their hours are numbered.
Optionally, whatever is toxic for humans in the atmosphere does not cause damage nor irritation to the skin and sensitive parts such as the eyes and nose, so that a person can be comfortable without the need for special suits or masks.
In case it helps, the planet I am sending people to die a slow death is Gliese 667Cc. I am going to handwave a few of its characteristics to make it more habitable (such as making it so that it is not tidally locked to its star).
So... What kind of atmosphere would allow you a grace period of a day or more before it kills you?