There is no labor law affirmatively or proactively requiring air conditioning. However, if you have made your employer aware of the problem (preferably in writing, so you can prove it) and if after having had warning and a chance to fix it, they do not, and if because of the lack of air conditioning, you suffer some injury or illness (and are able to prove that it was due to the lack of A/C), then they could be liable for your injury, etc. under a "negligence" theory: it is unreasonably careless to not fix a potentially hazardous condition when you have warning of it. So you can't make them provide A/C, but you can potentially hold them liable for the consequences of not doing so--though hopefully, there are no consequences.