Enhanced sense of smell. Preferably with a way to not immediately breath in the poisonous vapors. Also essential for information purposes: touch, sound, and sight will not provided sufficient information about the edibility of what one has in hand. Taste occurs a little late in the puzzle but might also help here.
Orientable Ears. Human hearing is good, but caves echo. Being able to alter ear shape and orientation would allow for specific sounds to be narrowed in on.
Small frames and overall lighter body structure. Smaller fissures can be traversed if you are smaller, and lighter builds are generally more flexible. This gives more options in moving around, and that can be life-saving when poisonous gas is around.
Chelating. Either by a behavioural method such as ingesting it, or by internal body processes. Its simply unavoidable that a person would not absorb at least some toxic chemicals, as such individuals who make them inert before they cause havoc on the more sensitive tissues will live longer.
Reflective Eyes. Not so much an evolution as retrovolution. Humans would need to obtain some more of the old night vision adaptations loost during the transition to day time living. This list also include larger pupils, and larger eyes.
Second eyelid. It might even be beneficial to re-evolve the second eyelids from the deep lizard past. This will provide some measure of protection and also means of eye-wetting without closing the eye. In such low-light levels one cannot say exactly when the elusive photon will reach the eye, best to just keep them open.
A Higher internal body-temperature tolerance. Either a fixed higher level locking them into living with a permanently higher body temperature (making them positively ravenous away from heat), or as an adaptive stress response (such as sweating) that can be more or less harmfully maintained for lengthy durations. Humans are already quite good at this, they can ancestrally hunt by forcing the hunted animal to experience heat-exhaustion first. But there are always ways to improve.
Loss of hair. Simply put who needs all that grooming. More seriously rock is more thermally stable and a simple fat lining is superior in such a stable setting. Need only look at naked mole rats. Also hair hampers the ability to cool actively, one of the reasons why humans can run so many creatures into the ground.