Working under hard conditions is not an excuse for not-fasting. Only travelers and sick people are allowed to postpone their fasting duties.
Allah the Almighty commands:
“Fast for a certain number of days. Whoever among you is sick or on a journey, should fast the same number some other days. (…) So whoever of you sees that month, should spend it in fasting. Whoever is sick or on a journey, should fast the same number some other days.” (2:184-185)
Working in the hot weather or under hard conditions is not an excuse for not-fasting. Unless if the person gets sick, then he can break his fast.
Please see also:
One of the virtues of fasting is to determine if a person will fulfill the commandments of Allah under all circumstances. According to the verses above, only travelers and sick people are provided convenience for fasting. Otherwise, one could have easily found an excuse for not-fasting in his/her own way and taken advantage of this provided convenience.