pH (historically, power of hydrogen or potential of hydrogen) is often described as a scale that goes from 0 to 14. If we look at the formula that defines pH, we’ll see that it has no upper or lower bound. pH = -log₁₀[H₃O⁺ (aq)] pH is equal to the negative logarithm (base 10) of the concentration of hydronium ions (H₃O⁺) in water. Sometimes H3O⁺ is written as H⁺, in the case with pH – it’s the same thing. H⁺ is called a hydron, a proton, a hydrogen cation, or a hydrogen ion. In the case of pH, since we’re looking at aqueous solutions, we assume the H⁺ is bound to a water molecule (H₂O). H₂O ..read more

You may have heard that: “SPF 30 absorbs 97% of the UV” or “SPF 50 absorbs 98% of the UV”. These numbers are from a math model and it’s quite simple! The math model is: 1 – (1 ÷ SPF) 1 divided by the sunscreen’s SPF, subtracted from 1. With an SPF 30: 1 – (1 ÷ 30) = 1 – (1/30 or 0.0333…) = 1 – 0.0333… = 0.9666… The ellipses (…) means repeating, the 666 in the decimal number 0.9666 repeats forever. For simplicity, we can round up 0.9666… to 0.97. We can then convert a decimal number to a percent by multiplying it by 100. 0.97 x 100 = 97% What’s the basis of this math model? The SPF of ou ..read more

Many of us just didn’t grow up with good sun protection education. I think a lot of us have forgotten that many of the bad effects caused by sun and UV exposure have only been recently well understood. While we’ve observed for a long time that sun exposure causes sunburn, the impact UVA has on skin’s appearance and photoageing are a relatively recent understanding and concern. Sunscreens marketed as an appearance maintaining essential are arguably modern. The first widely used “sunscreen” was Red Vet Pet. Used by American soldiers during WW II, it was a by-product of oil refining with a strong ..read more

Sunscreens and moisturizers with SPF are tested at a standardized density. That density is 2 milligrams of sunscreen per square centimeter of skin. If we want protection closer to what’s on the label, we should be using sunscreen or moisturizers with SPF at the density they’re tested at too. Most of us don’t know the surface area of our skin and most of us don’t know the density of our sunscreens either. This has led to techniques and recommendations, like using 2 or 3 finger lengths of sunscreen, using 1/4 teaspoon of sunscreen, or applying our sunscreen twice. These techniques are all meant ..read more

“25 percent of all people are going to be allergic to chemical sunscreens.” This statistic was published in a beauty magazine recently. Contemporary experiments and surveys have been done on the topic of sunscreens as an allergen. Here are the conclusions of some of them… “Of 23,908 patients patch tested, 219 (0.9%) had sunscreen coded as an allergen source. “ “Allergy to sunscreen represents a small proportion (< 1%) of allergic contact dermatitis reactions in North America” “[Allergic contact dermatitis] to sunscreen was found to be very uncommon (0.8%).” There are more papers on this ..read more

You might know that usually you shouldn’t mix tretinoin (all-trans retinoic acid) and benzoyl peroxide. Benzoyl peroxide is an effective acne treatment and works against acne-causing bacteria by oxidizing them. A major benefit of benzoyl peroxide is that it is unlikely to cause bacterial resistance. It may even reduce the risk of bacterial resistance to topical antibiotics when used alongside them¹. However, the oxidizing action of benzoyl peroxide is indiscriminate and can cause tretinoin to breakdown – likely along its tail where there are many sensitive carbon double bonds. Why might some ..read more

From your phone or screens? Probably not. When examining the results of studies looking at the effect of visible light, like blue light, we need to be really focused on the context. Studies that have shown a decrease in collagen, an increase in free radical production, or an increase in cell death…have been done on human skin cells in a petri dish. Those results will probably not translate to our skin. Our skin has more layers, including the epidermis. The epidermis contains a distribution of melanin. Melanin absorbs visible light like blue, green, and red light. Almost none of the effects obs ..read more

pH (historically, power of hydrogen or potential of hydrogen) is often described as a scale that goes from 0 to 14. If we look at the formula that defines pH, we’ll see that it has no upper or lower bound. pH = -log₁₀[H₃O⁺ (aq)] pH is equal to the negative logarithm (base 10) of the concentration of hydronium ions (H₃O⁺) in water. Sometimes H3O⁺ is written as H⁺, in the case with pH – it’s the same thing. H⁺ is called a hydron, a proton, a hydrogen cation, or a hydrogen ion. In the case of pH, since we’re looking at aqueous solutions, we assume the H⁺ is bound to a water molecule (H₂O). H₂O ..read more

You may have heard that: “SPF 30 absorbs 97% of the UV” or “SPF 50 absorbs 98% of the UV”. These numbers are from a math model and it’s quite simple! The math model is: 1 – (1 ÷ SPF) 1 divided by the sunscreen’s SPF, subtracted from 1. With an SPF 30: 1 – (1 ÷ 30) = 1 – (1/30 or 0.0333…) = 1 – 0.0333… = 0.9666… The ellipses (…) means repeating, the 666 in the decimal number 0.9666 repeats forever. For simplicity, we can round up 0.9666… to 0.97. We can then convert a decimal number to a percent by multiplying it by 100. 0.97 x 100 = 97% What’s the basis of this math model? The SPF of ou ..read more

Many of us just didn’t grow up with good sun protection education. I think a lot of us have forgotten that many of the bad effects caused by sun and UV exposure have only been recently well understood. While we’ve observed for a long time that sun exposure causes sunburn, the impact UVA has on skin’s appearance and photoageing are a relatively recent understanding and concern. Sunscreens marketed as an appearance maintaining essential are arguably modern. The first widely used “sunscreen” was Red Vet Pet. Used by American soldiers during WW II, it was a by-product of oil refining with a strong ..read more