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Within Kemetic ritual there are some measures of purity that must be taken. Ritual baths with natron (mixture of baking soda and kosher salt) are performed before entering the shrine. Sea salt should not be used. If you don't have a bath, then showering and pouring the natron and water over your head before the bath will suffice. The most minimal washing that should be done is rinsing out your mouth with natron mixed with water and washing your hands and arms (up to your elbows) with natron. And rinsing the natron off with water. A shorter washing is to rinse your mouth with natron and spit it out into the sink.

There are three items that are impure to all Netjeru. These are urine, feces and blood.

Urine and Feces
Both are products of waste. Both are impure. In incense, sometimes cow dung is used; to Hindus cow dung is sacred. It's best to ask for incense with no animal by products.

Blood
Blood flowing out of the body be it a cut or menstruation is impure. For menstruating women, this isn't that women are inferior to men or that women are less pure than men. Any blood flowing from anywhere out of the body is considered impure. For the blood taboo for menstruating women, offerings are allowed, but the Naos is NOT to be opened and formal ritual is NOT to be performed to an open statue. According to Blackman, in ancient Egypt menstruating women bathed in natron.

There can be situational exceptions to women menstruating if the Netjer within the opened statue being worshipped deems it so.

Bloodloss is the depletion of life energy; this is one of the more metaphysical reasons I have come across for why not to use it.

Loss of life energy would also explain why not to do formal ritual while you are ill or sick.

Offerings
There are some animals that are taboo for some Netjeru and aren't for others. For instance, Aset's taboo is pork. Set, on the other hand, loves pork and for Him would be pure. Most taboos with animals has to do with an animal being sacred to a particular deity. Also, eating an impure animal before going into shrine is considered impure. For example, if you are going to worship Aset, do not eat pork beforehand. Most of the time fish and seafood in general, are considered impure.

Some examples are:

• Aset: pork
• Wesir: pork and fish
• Amun: ram, goat
• Khnum: ram, lamb

Fabric
Some fabrics are impure such as silk (secretion of worms) and wool (hair of the sheep); they are impure because they come from animals. Linen is considered pure and so is cotton; fabrics coming from plants are pure.

Footwear
Shoes aren't to be worn in shrine because this brings outside impure forces into the sacred space. Shrine shoes or slippers used only during ritual can be used.

Shaving
For priests, shaving off body hair was important for purity in ancient Egypt. It was for health reasons also. For today, this may or may not be optional depending on the deity served.

Sex
Also for full priests celebrating a festival, having sex the night before is considered impure.

Birth
After a woman gives birth, she should not do ritual for fourteen days afterward. If a woman has a miscarriage, she should not go into shrine right afterwards.

Sources:

Discussing with Nemtet.

Alyward Blackman. Gods, Priests and Men. 1998.

Serge Sauneron. Priests of Ancient Egypt.