It depends on your state's labor laws and those of the U.S. Department of Labor. If you work in hazardous conditions and get hazard pay for example, or if you are covered by OSHA in any situation, you may be in luck. But to find out whether OSHA laws or other laws will help you, you need to contact those agencies and explain what is going on. If you are in a location wherein air coolers can be plugged in or safely used then there really shouldn't be an issue. I would think the employer would prefer to spend the money on air coolers than to get to pay workers compensation for any work related injuries as a result of working in extreme heat.