He works in a salt factory, and cannot help inhaling salt dust whilst fasting

Praise be to Allah.

If a person works in a salt factory and inhales salt dust, and finds the taste of salt in his throat, there is no blame on him for that and it does not invalidate his fast, because he cannot avoid this dust, and Allah does not burden any soul with more than it can bear. 

An-Nawawi (may Allah have mercy on him) said: 

Our companions (the Shaafa‘is) are unanimously agreed that if a person swallows a fly, or dust from the road or dust from sifting flour, without any deliberate action on his part, that does not invalidate the fast. Our companions said: He should not make the effort to keep his mouth closed if there is a lot of dust, or flour is being sifted, because that would cause undue hardship.

End quote from al-Majmoo‘ (6/359) 

Ibn Qudaamah (may Allah have mercy on him) said in al-Kaafi (1/441): 

Whatever cannot be avoided, such as swallowing one’s saliva, or dust from sifting flour, or dust from the road, or a fly that enters one’s mouth, does not invalidate the fast, because avoiding such things is not possible, and Allah does not burden any soul with more than it can bear.

End quote. 

It says in al-Mawsoo‘ah al-Fiqhiyyah (31/135): The fuqaha’ are unanimously agreed that the fast of a fasting person is not broken if dust from the road reaches his throat, so long as he did not do that deliberately, even if it is possible to avoid it by closing his mouth and the like when there is dust, because that causes undue hardship and difficulty, and because that is something that cannot be avoided, whether the fast is obligatory or supererogatory, whether the dust is small or great in quantity, and whether he is walking or not. 

According to the majority of fuqaha’, dust from the road and dust from sifting flour are the same, and it makes no difference whether the fasting person is one whose work environment is filled with dust or not, because it is something that cannot be avoided. The same applies to dust from plaster, for the one who is making it or selling it, and dust from flax, coal, barley and wheat. End quote. 

For more information, please see the answer to question no. 93821

And Allah knows best.