Embalming is ALMOST NEVER necessary.
No law in California requires it for any reason. No airlines require embalming for shipping a body unless the destination country requires embalming in order to accept the body.
You may be told that embalming is required in order to have an open-casket funeral, but this is a funeral home preference, not a legal requirement. Even for open-casket funerals, you may request that the body not be embalmed, and the funeral home should honor this request.
The process of embalming uses chemicals to retard the natural decomposition of a body. Note that embalming only slows down decomposition but does not stop it.
In the old days, prior to around 1910, embalming fluids contained arsenic, and leakage from from old cemeteries continues to pose a threat to groundwater because of it. However, modern formulations of embalming fluids do not contain arsenic and generally pose little threat to the environment because they quickly break down into harmless substances.
The main danger from embalming is to embalmers themselves, who have elevated death rates from arteriosclerotic heart disease, leukemia and cancers of the brain, colon, and prostate.
Embalming a body is an unpleasantly invasive process and family is not usually allowed to be present during the activity.