Looks like they reported a peak of 39C in the summer. Either way I figured that would still be pretty miserable, especially if it gets up around 100% humidity.

Assumed they would at least have their own air in the bits that didn't have aircon/ventilation while it was being built. They don't even need to do that anymore! The ventilation systems they used are as advanced and bespoke as the boring machines.

Because they were blasting too, they couldn't utilize full-face pressurization of the entire tunnel to maintain negative pressure to suck all of the fumes, dust, silicates, etc out like they would if it was only boring. That's 1-3kPa, "leaks are jets of air, can pull an airlock door closed hard enough to break bones" territory.

Instead, they have a bunch of dedicated supply and exhaust vents going to the surface (some up to 2m in diameter) and sets of connections between the two tunnels with huge axial fans. It allows them to selectively apply "slight" negative pressure to any of the individual segments when they need to clear them. 50Pa is ~10x what you encounter in a negative pressure highrise. It is described as a "constant slight breeze"

I found this short video on some of the safety features of the finished tunnel. It almost looks "too serious", like something out of a James Bond movie https://www.youtube.com/watch?v=I8trt96huf0