How Do Bath Bombs Work? The Science (With Video)

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lush-golden-wonder

Bath bombs are awesome balls of fizzy goodness, with some interesting science behind them! They were invented in 1989 by Mo Constantine, one of Lush’s founders. Bath bombs contain the chemical sodium bicarbonate, also known as baking soda, as their key ingredient.

This is the text version – scroll down for the video!

Some of you might remember that baking soda isn’t good for your skin because it’s a base, with a high pH. High pH (alkaline or basic) products disturb the skin’s acid mantle, which protects your living tissue from the environment, particularly bacteria, like acne-causing Propionibacterium acnes.

But don’t fret! The second key ingredient in a bath bomb is a solid acid, such as citric acid or tartaric acid (cream of tartar). This lowers the pH by reacting with the baking soda when water is added to the mixture. Unless the maker of the bath bombs has really messed up their proportions, the final pH should be reasonably neutral. Until the water dissolves the acid and baking soda and allows them to mix at a microscopic level, nothing happens.

Aside from neutralisation, the acid + base reaction with sodium carbonate also produces tiny bubbles of carbon dioxide gas, which is what causes the fizzing:

Citric acid + sodium bicarbonate → sodium citrate + water + carbon dioxide

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Do peeling gels really peel off my skin? (with video)

“Is all the stuff that rolls off with a peeling gel really my skin?“ This is a question I come across a lot! In case you’ve forgotten, a peeling gel is a popular type of cleanser in Asia. You start off with a watery gel like this, which you spread over your face (the one picture here is Laneige Strawberry …

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How to Use Comedogenicity Ratings (with Video)

rabbit-ear-test-comedogenicity

You’ve probably seen a comedogenicity chart like these ones (and the one further down the page) before, rating different ingredients on their ability to cause pimples.

Supposedly you check the ingredients list of your product against the comedogenicity list. If it has highly comedogenic ingredients, it will cause pimples, if it doesn’t, then it won’t. It’s simple, systematic and foolproof, right? Unfortunately, it’s not quite that simple…

This post goes into the science behind these comedogenicity ratings, and how exactly you should use them. It also comes in video form!

Click here for the video, scroll down for the rest of the blog post…

What does comedogenicity mean?

Comedogenicity is the tendency of an ingredient or product to clog pores. Ingredients are ranked on a scale:

  • 0 – completely non-comedogenic
  • 1 – Slightly comedogenic
  • 2-3 – Moderately comedogenic
  • 4-5 – Severely comedogenic

The numbers in comedogenicity scales come from studies performed by academics, published in peer reviewed journals – this usually means they’re somewhat reliable and valid. However, like with many other skincare claims “supported by the literature”, problems emerge when you dig deeper!

What’s wrong with the comedogenicity scale?

The problem is that the studies that produced the comedogenicity ratings don’t reflect real-world usage, for a number of reasons:

Tests aren’t done in real-world conditions

In an ideal world, we’d test every single product on every single person’s face, and develop a definitive comedogenicity rating list based on that. But this would be impossible – it would cost too much, there are too many products, and getting a lot of people to only use the one product and not change their daily routine for weeks or months at a time would be a mammoth task.

Instead, what’s used in most scientific studies is a model – a situation that mimics the real world, but is simpler to carry out and control. Think crash test dummies, dyed samples of hair, pouring blue liquid onto sanitary pads, patch testing potential allergens on your arm, testing bikes on a race track.

Most of the time these models work pretty well, but sometimes they don’t reflect the real world situation, so their results can’t be applied to everyday life (they have low external validity). In the case of comedogenicity ratings, the models don’t fare so well.

The most common rabbit ear test is flawed

The most common test for comedogenicity is the rabbit ear test, pioneered in cosmetics testing by two famous dermatologists, Albert Kligman and James Fulton, in the 1970s. This involves applying a substance to the inner ear of a rabbit, and waiting a few weeks to see if any clogged pores formed. Because rabbit ears are more sensitive than human skin, they reacted to comedogenic products faster, which was more convenient.

rabbit-ear-test-comedogenicity

Unfortunately, this also meant that there were lots of false positives, where ingredients that are non-comedogenic in humans would be found to be comedogenic in the hypersensitive rabbit model. Additionally, in the original tests, the scientists didn’t realise that there are naturally enlarged pores in rabbit ears. Some results counted these as acne, leading to even more false positives.

Related post: Purging vs Breakouts: When to Ditch Your Skincare

The most famous false positive is petroleum jelly (petrolatum or Vaseline), which was corrected in the late 1980s, but this was debated until the mid-1990s – that’s why the myth that Vaseline and oily products cause pimples is still so pervasive. This wasn’t the first time the rabbit ear tests were questioned – conflicting results were commonplace, and comedogenicity lists frequently disagreed with each other (and still do).

Related post: Is Mineral Oil Dangerous?

More recently in 2007, dermatologists Mirshahpanah and Maibach went so far as to say:

“[the rabbit ear] model is unable to accurately depict the acnegenic potential of chemical compounds, and is therefore only valuable for distinguishing absolute negatives.” – Mirshahpanah and Maibach, 2007

Tests on human subjects are also flawed

If rabbit ears don’t reflect what happens on human skin, then the obvious solution is to test on humans, right? Yes…but there are problems there too!

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Why you should protect your skin from UVA (and how) (with video)

You already know about SPF, UVB, sunburn and its relationship with skin cancer… but what about UVA protection? Newer research has really highlighted the important of UVA protection, so this post will tell you all about what UVA is, what it does and how to protect your skin against it (i.e. lots of nerdy sunscreen talk). The video version is …

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Don’t Use Lemon Juice on Your Skin

There are more DIY skincare recipes on the internet than you can shake a stick at, and most of them really, really like lemon juice. According to these DIY tips you should be slathering it all over your face and hair. Lemon juice smells nice, and it’s pretty cheap. But is it effective? And is it safe? Let’s talk about …

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Activated Charcoal in Skincare: The Science

activated-charcoal-cleansing-mechanism

Activated charcoal skincare products are pretty cool. They’re black, they’re sciencey-sounding and they’re said to suck dirt out of your pores like a magnet – what’s not to like? The reality is a little more complex than that…

(If you’re after the video version of this post, check it out here! To make sure you don’t miss a video from me when it goes live, click here.)

What is activated charcoal?

Activated_Carbon
Source

Activated charcoal is carbon soot that’s been treated to give it a sponge-like structure, with lots of holes. If you zoom into activated charcoal, it’s very jaggedy, giving it a huge surface area. It’s estimated that 1 gram of activated charcoal has a surface area of 3000 square metres, which is the same as 3 Olympic swimming pools, 7 basketball courts or 230 car parking spaces.

Activated_Charcoal-microscope
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This gigantic surface area is particularly handy for soaking up substances. You can see how effective it can be for purifying water in the picture underneath, where activated charcoal’s grabbed onto all the red dye in the glass on the left (it can also be seen with the orange fizzy drink in the video).

Activated_carbon-red-dye-adsorption

In medicine, activated charcoal is mostly used in poisoning cases, where a large dose is fed to the patient alongside other treatments. A lot of the poison sticks to the charcoal instead of absorbing into the body.

Since it’s a default treatment for soaking up ACTUAL TOXINS in poisoning, lots of people eat/drink activated charcoal for “detox” purposes, which sounds like it could work (although if you read my other blog, you’ll know that detox is a scam). You might be wondering, how does activated charcoal tell the difference between good things and bad things? The answer is… it doesn’t.

How does activated charcoal work?

At a molecular level, things are sticky. It’s why the wax in a candle stays together as one big clump instead of splaying everywhere, and why you have to put in a whole heap of heat to get water molecules to separate and turn into steam. This stickiness is known as intermolecular forces – interactions that stick molecules together. Without them, everything would be a gas.

There are a few types of intermolecular force – the one that activated charcoal uses is called dispersion forces. Absolutely every substance has dispersion forces, whether it’s a vitamin or a poison.

Remember the large surface area that activated charcoal has? This means there’s a lot of space for things to stick to. And since everything can form dispersion forces, activated charcoal actually soaks up all sorts of things, including nutrients like vitamins, meaning you don’t get the full health benefits of your food.

Activated charcoal can stick to medications as well, if they’re still in your digestive system. This means that you might not be getting the right dose.

In a poisoning situation, you’re probably not worried about whether you get enough vitamins, but in everyday life it’s not such a good idea to prevent your body from taking up random nutrients and medications on a regular basis. So don’t make activated charcoal part of your regular diet!

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What is thermal water and how does it work in skin care?

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If you walk into a pharmacie in France, you’ll immediately bump into a giant display of thermal water spray cans. A whole host of French skin care brands like Avène, La Roche-Posay, Uriage and Vichy sell thermal water sprays. What is it, what’s in it, what does it do and how is it different from regular water?

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What is thermal water?

Thermal water comes from hot springs. The water in these hot springs come from deep in the ground, where it’s heated by geothermal activity (the Earth’s natural heat which also causes lava to be molten).

What’s in thermal water?

It’s mostly water, of course, but it isn’t “just water in a can”! As the thermal water rises to reach the spring, it passes through rocks and soil which dissolve to add minerals to the water. The mineral content of a particular thermal water depends on where it comes from. The minerals include the ones found in your skin’s natural moisturising factor (NMF), like chlorides, sodium, potassium, calcium, and magnesium.

Here are the compositions of the 4 most popular thermal waters (source: Bacle et al., Int J Dermatol 1999, brand marketing materials). There’s some variation between batches of course, since it’s a natural mixture.

Composition (mg/L or ppm)AvèneLa Roche-PosayVichyUriage
Total dry residue207444512011000
Sodium4.81018602360
Potassium0.72.299.645.5
Calcium42.7140150.6600
Magnesium21.24.912.3125
Chlorides5.4253573500
Bicarbonates226.73964776.3402
Sulfates1430Trace2862
Nitrates1.4TraceTrace< 100
Silica (SiO2)10.630-42
Selenium-0.060--
Strontium-0.260--
Zinc-0.022-0.16
Copper-0.005-0.075
Manganese---0.154
Iron---0.015
pH7.46.97.006.77

As you can see, the composition varies a fair bit, with Uriage, the thermal water with the highest mineral content, being 55 times more concentrated than Avène, which has the lowest mineral content.

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Residue remaining after room temperature evaporation of 10 mL of Uriage Thermal Water in a 250 mL beaker.

“-” in the table means no data, since different thermal water brands like to highlight different aspects of their water. La Roche-Posay talks a lot about the selenium content of their water, while Uriage emphasises the high calcium concentration. Avène talks a lot about the 2:1 ratio of calcium and magnesium in their water.

There’s also nitrogen gas inside the can that acts as a propellant, to push the water out as a spray. Paula’s Choice writes that the nitrogen “can generate free-radical damage and cause cell death”, which luckily isn’t true, since nitrogen gas (N2) make up 78% of the air we breathe! It’s actually very unreactive, so unreactive that it’s commonly used in laboratories to flush out more reactive things like oxygen and water. (The papers cited in the Beautypedia article actually involve chemicals that just contain nitrogen atoms, not nitrogen gas itself.)

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Why You Should Never Straighten or Curl Wet Hair

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hair-straightener-wet-dry

I’ve been straightening and curling my hair wet/damp for years, despite all the popcorn-sounding fizzes and crackles – but some science I’ve recently read have made me do a complete 180. It turns out wet and dry hair respond to extreme heat very differently – and wet hair cops it far worse, and not just because it’s more fragile.

First, a little refresher on hair anatomy. Hair has three main parts:

  • the medulla – boring central core, actually non-existent in light and fine hair
  • the cortex – middle layer, responsible for colour, texture and most of the strength of hair
  • the cuticle – protective outermost layer, made of overlapping cells like roof shingles or fish scales, shiny

cross-section-hair-structure

The cuticle lays flat, but water can get in between the gaps. When you wet healthy hair, it can actually absorb up to 30% of its own weight in water into the inner spongy cortex (more if it’s damaged hair).

An average hair straightener heats up to 185-230 °C. Curling irons are a little cooler, at 95-200 °C. They’re both well above the normal boiling point of water, which is 100 °C. (Temperatures in Fahrenheit are straighteners 365-446 °F, curlers 203-392 °F and boiling water 212 °F, for any readers from non-metric countries like the US, Liberia or Myanmar.)

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C’mon USA, switch already. I believe in you.

What happens when the wet inside of hair gets heated well beyond its boiling point? Well, it’s not too dissimilar to popping corn…

Source
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When water turns from liquid to gas it expands. When it’s heated strongly it expands rapidly. Since it’s confined in the cortex by the cuticle, it has to bust out. That’s right – the water explosively evaporates, shattering whatever’s in its path, which happens to be… your hair.

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From Gamez-Garcia, J Cosmet Sci 2001, 62, 109-120

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