Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.   Wa alaikum assalam wa rahmatullahi wa barakatuh.   As to what follows:   In response to your question, we say:   If  fasting while doing hard work involves hardship which causes death, or sickness which makes you weak and disturbs your life, then this takes the ruling of sickness, because the fear of sickness is an excuse which permits the breaking of fast. That is why a person who lives normally but suffers from lung failure or diabetes is permitted to leave fasting if it will cause extreme hardship for him.   However, hardship is a relative issue which is between the servant and Allah.  Thus I say to the questioner, “If the hardship  involved in fasting while working in a hard job reaches an extent where you are afraid that you may die or get sick then it is permitted for you to leave fasting and your condition will be like that of a sick man, or those whom Allah the Exalted has excused. However, if there is hardship which you can avoid by wetting your head, or grazing the animals early before the intensification of heat, then that is better”.