They work in difficult jobs such as smelting metals

One of the basic facts of Islam, which no Muslim has any excuse for not knowing, is that fasting the month of Ramadan is obligatory upon each person who is accountable, and it is one of the pillars of Islam. So every person who is accountable must strive to fast it in order to fulfil the obligation that Allah has enjoined upon him, hoping for His reward and fearing His punishment, without forgetting permissible earnings in this world and without preferring this world to the Hereafter. If there is a conflict between doing the acts of worship enjoined by Allah and working to earn a living in this world, then he must strike a balance between them so that he will be able to do both. In the example mentioned in the question, he should make the night his time for working to earn his living in this world, and if that is not possible then he should take a leave of absence from his work during Ramadan, even if that is without pay. If that is not possible then he should look for other work where he will be able to fulfil both duties.  

His worldly interests should not affect his Hereafter interests. There are lots of jobs and the ways of earning a living are not limited to this kind of difficult work. By Allah’s Leave, the Muslim will never want for permissible ways of earning a living which will allow him also to do the acts of worship enjoined upon him by Allah. Allah says (interpretation of the meaning): 

“And whosoever fears Allah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).

If we assume that he cannot find any work other than that mentioned, which causes him hardship, then he should flee for the sake of his religion from that land to another land where it will be easy for him to fulfil both his religious and worldly duties, and where he can cooperate with other Muslims in righteousness and piety, for the earth of Allah is spacious. Allah says (interpretation of the meaning): 

“He who emigrates (from his home) in the Cause of Allah, will find on earth many dwelling places and plenty to live by” [al-Nisa’ 4:100] 

“Say (O Muhammad): O My slaves who believe (in the Oneness of Allah Islamic Monotheism), be afraid of your Lord (Allah) and keep your duty to Him. Good is (the reward) for those who do good in this world, and Allah’s earth is spacious (so if you cannot worship Allah at a place, then go to another)! Only those who are patient shall receive their reward in full, without reckoning” [al-Zumar 39:10] 

But if he cannot manage to do any of these things and he is forced to do the kind of difficult work mentioned in the question, then he should fast until he feels that he cannot bear to continue fasting, and then eat and drink enough to keep him going, then he should refrain from eating and drinking. And he has to make up the missed fasts on days when it is easy to fast. 

And Allah is the Source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions.