Toxins: Shampoos and other cosmetics contain a lot of chemicals which can be harmful in long term usage. The list includes parabens, which emerging studies are linking to increased rates of breast cancer and elevated estrogen levels. Not to mention sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES), an industrial degreaser which is actively toxic.
Crude Oil: Shampoo also contains propylene glycol, which is a petroleum product. I think some day humanity will look back and be amazed that we not only powered our vehicles with petroleum, we also used it in just about every non-food product (and some food products). If you're trying to reduce your gas consumption, you could start with your shampoo!
Waste: Think of all the plastic that gets used in all those shampoo bottles, which all go straight to the dump! Sure, some municipalities are able to recycle shampoo bottles, but these are few and far between. And the type of plastic used in shampoo bottles is not in high demand on the recycling market.
Ground Water: Shampoo gets washed down the drain and into the water table. The amount of parabens, SLS, SLES, and propylene glycol may not be enough to poison an adult human being outright. But it's a different story for smaller life forms including beneficial bacteria, fish, insects, and amphibians.
If you have curly hair, shampoo is actually detrimental to the hair shaft. It has the opposite effect from intended, causing your hair to look frizzier and less glossy. If you have lightweight hair, shampoo and conditioner weigh down your hair and make it look worse.
Is there any reason TO use shampoo? I keep a bottle around and use it when my hair gets truly dirty, maybe once a month, usually due to some unfortunate gardening incident. Once I finish this last bottle (in 2-3 years, at this rate) I'll just use my regular bar soap in these cases instead.
What to use instead of shampoo? Baking soda of course - is there anything it can't do?
I use an old plastic sports bottle (having converted to metal or glass drinking containers) to mix the concentrate. Drop 3-4 tablespoons of baking soda into the sports bottle and top it off with water, then shake vigorously to mix.
When I'm in the shower, I squirt about ¼ cup of the baking soda concentrate mixture into an old plastic margarine tub. Then I add hot water from the shower to top off the container. (This way you never have to pour cold baking soda water down your back!)
Massage the mixture into your scalp and let it sit for a few minutes, then rinse it out.
About once a week I swap in a white vinegar treatment. Same deal - vinegar and water mixed about 10:1 in a sports bottle, pour a bit into the margarine tub and mix with hot water from the shower. Rub into your scalp, let it sit, then rinse it out.
Try it for a week and adjust as necessary. If your hair gets too oily, use the vinegar rinse less often (every 10 or 12 days). If your hair gets too dry, use the vinegar rinse more often (every 2nd or 3rd day). You'll be amazed at the results!
Creative Commons-licensed image of No 'Poo hair courtesy of Flickr user paxye