In the end, the decision was that the first to make contact, as well as a few of the older members of the group who were past their child-raising age and already had trained their successors, would be the first to go outside and make sure it was safe.
The Team Leader sent one of their members ahead, following the aliens' directions on where the open door to the lower levels was so that they could clear the tunnel and purge the polluted air that was trapped in it.
It would ruin their credibility if the first thing that happened was that the aliens began to choke on toxic fumes the moment that they left their enclosure, and it would likely stop them from going outside at all for many years, even though the sky was clear.
[Team Leader, the exit tunnel has been purged and fully sanitized. The air quality in it matches the outside world, and you are safe to proceed.]
