Lacking the inhibition of a safe amount of preservative, microorganisms can grow unchecked and create a genuine health risk for anyone using them. Another potential effect of significantly diminished preservative concentration is degradation of the molecules in the product itself via oxidation from environmental exposure. This can cause development of noxious odors or the formation of potentially dangerous degradation products. At best, it can simply render your product ineffective.
The risk of your being unwittingly exposed to organisms or degradation products is most high when the solution is already cloudy due to the presence of an oil component (such as in a conditioner or styling product), which obscures your ability to observe growth at an early stage. Also, due to the presence of fragrance, one might not detect spoilage by smell in the early stages, either. By the time you are able to detect a problem, you could possibly have been applying a contaminated product for a few days.
My recommendation if you like to use diluted conditioner or other products on your hair is to do one of the following (and always with clean hands, boiled water, and a sterilized container):
- Make up a small amount of solution daily or every other day.
- Make up enough solution to last for one week and store it in the refrigerator when not in use.
- Make up enough solution to use for 1-2 weeks and add either natural or synthetic preservative drops to it in an amount specified in the literature. (Synthetic preservative mixtures, such as Kathon, can be purchased from a chemistry or cosmetic ingredients source, and natural preservatives such as grapefruit seed extract can be found at most health food stores). I would personally still store this mixture in the refrigerator when not in use and discard after a couple of weeks.
The same basic rules would apply when making a natural homemade concoction to treat your hair. If there will be water involved (in other words, it is not a pure oil product), then proper precautions regarding sterility and preservation should be taken to ensure your own safety.