My job is too physical and work load is increasing?

I am doing a job that is physical and requires me to work in very hot environment. I have been given extra duties that require me to be in a hot environment even more. I have discussed this with my supervisor and am being told that it is part of my job. When I started this job It was never disclosed to me that I would be given more physical work in this type of environment, I have asked for monetary compensation and am told that it is not available... I am 51 and a single mother of two kids that need me more. I can not do this extra physical work and raise my children. I am fearing a new job search with the way the unemployment is going..... HELP.........