What counts as a cosmetic, and skin and hair types
A cosmetic is any product applied to the outside of the body (skin, hair, nails, lips, teeth) to clean, perfume, protect or change its look. Medicines that treat disease are not cosmetics.
Skin types depend mainly on how much oil (sebum) your glands make: oily (shiny, larger pores), dry (tight, flaky), combination (oily forehead, nose and chin; dry cheeks), normal, and sensitive (reacts easily). Hair can be straight, wavy, curly or coily, and oily or dry at the scalp.
Daily care is simple: wash with a mild cleanser, moisturise, use sunscreen outdoors, eat and sleep well. Healthy skin is slightly acidic (pH about 5), so very alkaline soaps can dry it.
Inside a cream: emulsions and ingredients
An emulsion is a mix of two liquids that normally separate, like oil and water. An emulsifier molecule has a water-loving (hydrophilic) head and an oil-loving (hydrophobic) tail. The tails sit in the oil drop and the heads face the water, so drops stay spread out.
- Oil-in-water (O/W): light lotions and day creams; feel non-greasy.
- Water-in-oil (W/O): cold cream, rich night creams; feel greasy, lock in moisture.
Typical ingredients: water, oils/waxes (softeners, emollients), humectants like glycerol that pull water into skin, emulsifiers, thickeners, preservatives to stop germs, fragrance and colour. On a label ingredients are listed from the largest amount to the smallest (INCI names).
Shampoos and cleansers use surfactants, the same idea as soap: they grab grease and let water wash it away. Hair dyes and colours use oxidising chemistry (often hydrogen peroxide with ammonia) to open the hair and form colour inside.
UV light and sunscreens
Ultraviolet (UV) light has shorter wavelengths than visible light and carries more energy per photon.
- UVB (280–315 nm): stopped mostly in the epidermis; causes sunburn and helps make vitamin D.
- UVA (315–400 nm): reaches the dermis; causes tanning, wrinkles and DNA damage. It passes through clouds and window glass.
Sunscreen filters: organic (chemical) filters absorb UV and release it as a little heat; mineral (physical) filters like zinc oxide and titanium dioxide reflect and scatter it.
SPF (sun protection factor) is about UVB. Fraction reaching skin ≈ 1/SPF: SPF 15 blocks about 93%, SPF 30 about 97%, SPF 50 about 98%. Look for broad spectrum (or a UVA mark / PA+++) for UVA. Use about 2 mg per cm² (around a teaspoon for the face and neck) and reapply every 2 hours or after swimming.
Antioxidants, safety, cost and the environment
A free radical is an atom or molecule with an unpaired electron, so it is very reactive. It can damage cell membranes, collagen and DNA. Antioxidants (vitamin C, vitamin E, green tea compounds) give it an electron and stop the chain reaction. They are also added to creams to stop the oils going rancid.
Use responsibly: read the label and expiry date (the open-jar symbol, e.g. 12M); do a patch test (a little on the inner arm for 24–48 h) before new products and hair dye; never share eye or lip products; stop if you get redness or itching.
Cost: compare price per gram or per use, not per pack. A simple cleanser + moisturiser + sunscreen covers most needs; salon treatments cost more and should be chosen for a real need.
Eco-friendly choices: avoid plastic microbeads, choose refills and recyclable packs, use less product, and pick reef-friendly sunscreens where possible.
Try it
Shake 2 spoons of oil with 2 spoons of water in a jar: they separate in a minute. Now add a little liquid soap or an egg yolk (both contain emulsifiers) and shake again: you have made a cloudy emulsion that lasts much longer. Then read the ingredient list on two creams at home and find the emulsifier, the humectant (glycerin) and the preservative.
Key formulas and definitions
- Emulsion = droplets of one liquid spread in another, held by an emulsifier
- O/W: oil in water (light lotion); W/O: water in oil (cold cream)
- UVB 280–315 nm (burns); UVA 315–400 nm (ages)
- Fraction of UVB reaching skin ≈ 1 / SPF
- % blocked ≈ (1 − 1/SPF) × 100
- Healthy skin pH ≈ 4.5–5.5
Worked examples
1. What percentage of UVB does SPF 20 block?
Fraction in = 1/20 = 0.05 = 5%. Blocked = 100 − 5 = 95%.
2. Without sunscreen your skin reddens in 10 minutes. With SPF 30 applied properly, about how long?
About 10 × 30 = 300 minutes in theory. In real life sweat and thin layers lower it, so reapply every 2 hours.
3. A 50 g cream costs ₹200 and a 150 g one costs ₹480. Which is cheaper per gram?
₹200 ÷ 50 = ₹4.00/g; ₹480 ÷ 150 = ₹3.20/g. The big pack is cheaper per gram (if you will use it before it expires).
4. Why does a cream with oil and water need a preservative?
Water lets bacteria and moulds grow. A preservative stops them so the cream stays safe after opening.
Common mistakes
- Thinking SPF 50 blocks twice as much as SPF 25. It blocks 98% vs 96%; the bigger gain is applying enough and reapplying.
- Believing you do not need sunscreen on cloudy days or indoors near windows. UVA passes through clouds and glass.
- Thinking 'natural' means always safe. Plant oils and essential oils can also cause allergies; patch test anyway.
- Comparing pack prices instead of price per gram or per use.