Praise be to Allah.
A person is not affected by whatever gets onto him of dust from impure substances (najaasah) or clothes that have been contaminated with impure substances, for several reasons:
- This dust that comes off the clothes does not have any of the qualities of impure substances.
- This dust is usually dry, with no moisture or dampness in it. Impurity (najaasah) cannot be transferred except in the presence of moisture.
- With regard to dust such as this, even if we assume that it carries any impurity, the impurity that it would carry is very little, and trying to avoid that is usually too difficult.
Shaykh al-Islam Ibn Taymiyah said: Things that are too difficult to avoid are overlooked, such as smoke and dust that have been transformed and originated from impure things, just as other things are overlooked that are too difficult to avoid, such as mud and dust from the streets. End quote.
Al-Fataawa al-Kubra (5/312).
Al-Khateeb ash-Sharbeeni said: A small amount of smoke coming from impure things and dust from animal dung, and the like, which may be carried by the wind as tiny particles, is overlooked. End quote.
Mughni al-Muhtaaj (1/128).
Al-Mirdaawi said: A small amount of smoke or dust or vapour from impure things may be overlooked, unless it shows one quality of the impure substance. This is the correct view according to our madhhab. Abu’l-Ma‘aali said that dust from impurities is overlooked in all cases, and he did not restrict it to small amounts, because there is no way to avoid it.
It says in al-Furoo‘: And this is the sound view. End quote.
For more information, please see the answer to question 164471.
And Allah knows best.