If a person works in a salt factory, what should he do, as there is a great deal of salt dust and he cannot avoid it? Is his fast rendered invalid if he inhales it or if there is the taste of salt in his throat?

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.