Question 650: Assalam o alaikum. As in many countries have out door laundry shops where people come and wash their dirty clothes, which can have stains of najis blood or other things. So can we use those washing machines for our laundry? Actually, I want to know the Rules on using washing machines when you have doubts.
Answer 650: Generally, a well-known religious law says: “Everything is ritually pure for you unless you come to know that it is ritually impure.” This law declares everything to be pure unless one becomes sure a particular item has become impure. And as long as you are not sure that it has become ritually impure (najis), it is to be considered pure and you can apply all the rules of purity to it without any hesitation or doubt.
The washing machine can purify najis things if the pure water covered the entire impure area twice (in case it had become impure by urine and even if the water was connected to kurr source as an obligatory precaution) or just once (when it had become impure by other elements); and that the water was removed from the clothes by wringing or other similar method [i.e., spinning of the machine] if it was qalil.
Therefore, as long as you are not sure and just have conjecture that the dirty clothes hasn’t been purified as per the religious requirement or because there is a possibility that najis drops have remained inside the machines and then might reach your clothes, there would be no problem in using those washing machines for your laundry. Otherwise, if you come to know about it you wouldn’t be allowed to use them.
For further information on this issue, please read the following answers:
 . Tawzih al-Masael of maraja, Vol. 1, Pg. 85, issues: 121-123;The official website of the office of Sayyid Sistani (ha), Q&A: Najasat.