- Najasa refers to impure substances that Muslims must avoid and wash off after coming into contact with them. God says: Purify your raiment (74:4) and: God loves those who repent and who purify themselves (2:222).
- Animals that died naturally (e.g., not killed in the Islamic manner) are impure, as is anything cut off a live animal. However, dead sea animals and those that have no flowing blood (e.g., bees and ants) are not impure. The bones, horns, claws, fur, feathers, and skin of dead animals, except for pigs, are pure.
- Any blood that flows from a person's or an animal's body (e.g., blood from a killed animal or menstrual blood) is impure. However, blood that remains in the veins is permissible. Also, any blood that remains in edible meat, livers, hearts, and spleens is not impure, provided that the animal was sacrificed in the Islamic way.
- A person's vomit, urine , excrement, wadi (a thick white secretion discharged after urination), mazi (a white sticky fluid that flows from the sexual organs when thinking about sexual intercourse, foreplay, and so on), prostatic fluid, and sperm is impure. However, according to some, sperm is not impure but should be washed off if it is still wet, and scratched off if it is dry. Any part of human flesh is impure.
- The urine, saliva, and blood of all animals whose meat is prohibited, and the excrement of all animals except birds whose meat is allowable, are impure.
- The excrement of poultry (i.e., geese, hens, ducks) is impure.
- Pig and alcohol are impure.
- Dogs are considered impure. Any container that a dog has licked must be completely washed and sterilized. If a dog licks a pot that has dry food in it, what it touched and what surrounds it must be thrown away. The remainder may be kept, as it is still pure. A dog's hair is considered pure.
- The impurities mentioned are considered "gross impurity" (najasat al-ghaliza). Any amount of them contaminates whatever it touches. However, if it is on person's body or clothes when he or she is praying, or on the ground or mat where he or she is praying, its amount is taken into consideration. Any solid filth weighing more than 3 grams, and any liquid more than the amount that spreads over a person's palm, invalidates the prayer.
- The urine of horses and domestic or wild animals whose meat is allowed to eat is weak impurity (najasat al-khafifa). When more than one-fourth of a limb or one-fourth of one's clothes are smeared with it, the prayer is in-validated.