The Torah commands us to be kedoshim (holy, or set apart), requiring purity in what we eat, how we speak, what we wear, and how we behave. Tum'a is spiritual impurity generally resulting from transitions from life to death (even in a small measure; for instance—sleep or cutting nails). Tum'a inhibits or blocks us from achieving holiness.
Since we do not have a “red heifer” with which to make purifying water solutions, all people are considered to have some level of tum'a today.
Although there are three reasons to ritually wash hands--to add kedusha; to remove tum'a; and to remove dirt--tum'a normally has nothing to do with physical dirt. There are many types and levels of tum'a, with no exact progression. The following guideline is approximately in descending order from most impure to least:

Sources of the Different Levels of Tum'a

• Dead body (touching or being in same building with a dead body);
• Cemetery;
• Carcass of any dead animal not slaughtered by shechita;
• Women during and after menstruation or after childbirth (but before they immerse in a mikva);
• Sleep;
• Possibly a bathroom;
• Your hands' transferring tum'a to wet food;
• Your hands' transferring tum'a to bread;
• Food from under a bed on which someone slept;
• Intercourse or seminal emission;
• Having your beard, hair, or nails cut;
• Leather shoes;
• Touching body parts.

Depending on the level of tum'a, purifying may require:

• Washing your hands by the Three-Times Method.
• Washing your hands by the One-Time Method.
• Immersion in a mikva. OR
• Sprinkling with water that had been treated with ashes from a red heifer (which we do not have now).

Nail clippings, even from children and non-Jews, have ru'ach ra'a and need to be disposed of. The simplest way is to flush them down a toilet or wash them down a drain (but don't put them into the garbage).

Some foods (such as onions, garlic, and scallions/green onions) will pick up ru'ach ra'a if:

• Peeled, AND
• Cut at both ends, AND
• Left overnight in a home, AND
• Raw (OR cooked) and not mixed with other foods, spices, or salt.

Note Under the above conditions, the food will pick up ru'ach ra'a even if stored in a sealed container after being cooked. However, if such an onion (raw or cooked) that is peeled and cut at both ends is mixed with something else--whether other foods, oil, spices, or salt--the onion will not pick up ru'ach ra'a.

Problematic foods:

• Onions,

• Garlic,

• Eggs (hard boiled or raw once they are out of their shell).

Not a problem:

• Unpeeled onions

• Cut and open lemons or other produce(excluding onions, garlic, and eggs).

• Raw onion, garlic, or eggs that were cut or peeled in a commercial facility and remain uncovered overnight.

Note You may not use an onion that has been left overnight under any circumstances (even if wrapped in clear plastic wrap, put into the refrigerator, etc.):

• That has been cut at the top and bottom, and

• Whose brown layer has been removed.

Note Spring/ green onions also attract ru'ach ra'ah, but only if you cut off all of the green and also the roots.

Note If the onion or garlic had been peeled and cut at both ends but you sprinkled some salt on it, then you may use it even if it has been left out overnight.

Bal Tashchit May Override Custom
You may not destroy things in the world for no purpose. You may not waste anything (bal tashchit) that has a use, but you may use it for a purpose. Bal tashchit overrides customs and suggestions of what are good behaviors or actions.
Killing Creatures that Harm
You may kill any animal, bird, or other living creature that bothers, injures, or endangers people or destroys property (as long as it is legal by the laws of the local country or area). This includes animals that eat your food or produce.