On average, a typical person goes through 3 bottles of shampoo in a year. In fact, shampoo is the leading product in the hair care industry.
So, why is shampoo so widely used? What does shampoo do? How does it really work to clean the hair?
Allow me to break it down for you.
What does shampoo do?
Shampoo cleans the hair by removing grease, dirt, and debris from the scalp and hair strands.
They also nourish the hair with antioxidants and oils all without causing harm or irritation to the hair, scalp, and other body parts.
Some types of shampoo also work to soften the hair and make it more manageable.
How does shampoo work?
Shampoos are made up of various components which work together to clean the hair.
Here is a breakdown of how each shampoo component works:
Surfactants remove sebum and other particles in hair, aid form creation, and support the viscosity; all while having as mild effects on the hair, skin, and eyes as possible.
Surfactants are mainly categorized into primary and secondary surfactants. 
Primary surfactants do the heavy lifting. They occur in more concentrated amounts and accomplish a majority of functions.
Examples of primary surfactants include:
- Sodium lauryl sulfates
- iethanol lauryl sulfate
- mono ethanol laurysulphate
- ammonium lauryl sulfate.
Secondary surfactants work to supplement the work done by primary surfactants.
Secondary or co-surfactants include dialkylsulphosuccinates, monoalkyl sulphosuccinates, and fatty acid alkanolamides. 
2. Conditioning agents
Once the hair has been cleansed, conditioning agents keep the hair soft, moisturized, shiny, and detangled.
Conditioning agents come in seven classes that include: cationic surfactants, occlusive, humectants, emollients and oils, polymers, silicones, and proteins.
These agents work to condition the hair in 3 main ways:
- Use static charge, as is in the case of cationic surfactants which are positively charged, to attach to the hair which is negatively charged. As a result, they stick to hair long enough to provide much-needed moisture.
- Draw moisture to the hair strands since they have properties that can attract and hold water, just like a sponge. This is true for humectants
- Lubricate the hair surfaces to make hair more flexible and shiny as is the case with emollients. Occlusive work in a similar manner but goes ahead to create a thin coating on hair strands that blocks water loss into the atmosphere from both hair and scalp.
3. Anti-dandruff agents
Anti-dandruff agents are active ingredients present in shampoos to fight excess oil production and clamping of dead cells in hair which majorly contribute to dandruff formation. Specific ingredients include:
- Sulfur: has antibacterial and antifungal properties and prevents dead skin cells from clumping together.
- Zinc pyrithrione: Kills fungi and bacteria and improves oil production
- Coal tar: Reduces the speed at which scalp cells die ultimately reducing flaking
- Salicyclic acid: Reduces scaling and prevents skin cells from clumping together to form flakes
Other active ingredients include ammonium compounds such as benzalkonium chloride, cetrimide, and phenol derivatives.
4. Pearlescent agents
Pearlescent agents work to improve the look and feel of shampoos. This is achieved by thickening the product, making it less transparent and more opaque, and adding color and fragrances.
The main pearlescent agent used in shampoos is Glycol Stearate. Some of its functions include:
- Making the shampoo more opaque by adding a white/shimmery appearance.
- It also acts as a conditioner and emollient to smoothen skin and scalp tissue.
- Acts as a stabilizer to hold oil particles to the water solution. This prevents the separation of the product and extends its shelf life.
- Builds the viscosity of shampoo making it thicker.
Some other common pearlescent agents include 4-methyl-7-diethylaminocoumarin and 4-methyl-5, 7-dihydrocoumarin.
In case you have used hardened water to wash your hair, you may have noticed some scum or a film forming on your hair.
Sequestrants in shampoos prevent scum from coating hair which makes the hair have a dull appearance. Commonly used sequestrants are sodium triphosphate and EDTA.
These work to increase the shelf life of hair cleansing products. Some common components include p-hydroxybenzoic acid and itsmethyl ester, phenyl mercuric compounds, and formaldehyde.
How long should you leave shampoo in your hair?
Shampoo should be massaged into the hair for approximately 1 to 3 minutes before rinsing.
Hair professionals recommend scrubbing your scalp alone for at least three minutes. This ensures that the build-up present on the hair and scalp is adequately removed.
How do you know if a shampoo is working?
A good shampoo will make your hair and scalp feel clean and not oily or greasy.
Your locks and curls will be lively and not dull. Look out for a scalp that turns dry or becomes irritated after washing your hair. This may be an indication that the shampoo is too drying for your hair. In this case, consider a more hydrating shampoo.
A shampoo that leaves your hair feeling greasy or with oily patches might also not be cleansing enough. Tangled hair is also a sign that your shampoo is not suited for your hair type. This may be caused by the shampoo interfering with your hair`s moisture absorption abilities.
How much shampoo should you use?
The amount of shampoo necessary for hair varies according to the hair`s texture. Fine hair will generally require less amount of shampoo, approximately a nickel-sized.
Thicker hair will require double the amount, a size of 2 quarters. Any hair with medium texture will require any size in between 1-2 quarter-sized amounts.
In the case that the shampoo does not lather well, hair professionals advise that this might be remedied with double cleansing using the required amount of your textures.
This is because in most cases, the minimal lather is a result of excess build-up rather than a little amount of shampoo.
How long does shampoo last?
When it comes to the shelf-life of shampoo, they typically last anywhere from 12 to 24 months depending on the specific brand. The period by which shampoo should last reduces after the shampoo is opened.
This information is usually available on the shampoo product so be sure to check it before you purchase.
In case this information isn’t available, you can tell a shampoo has gone bad if it doesn’t cleanse as well as it used to over time.
Other factors to check include an odd smell, a change of color, and a clumpy appearance.
On usage, depending on frequency of washes, volume of hair and lather properties, shampoos can last anywhere from 3 to 12 months.