Sunday, September 3, 2023

UV Protection Provided by Hair

How much UV light from the sun does your hair block out? Maybe more - or less - than you thought. It's really variable from person to person and depends on a number of conditions!

If you spend time in the sun - whether you are an outdoors worker, do outdoor sports, gardening, hiking, you need information to make good decisions about scalp protection in the sun.

Your scalp - especially the top is always facing the sun. Especially during the strongest rays of the day. 

People with very light skin (Fitzpatrick Phototype 1, 2) are at highest risk of melanoma and non-melanoma skin cancer from that sun exposure. People with deeper skin tones (i.e. Fitzpatrick Phototype 3 and greater) may be susceptible to hyperpigmentation with UV exposure.

All of us are at risk of skin dehydration, irritation, and worsening of existing scalp conditions with UV exposure. Even if skin cancer is not a concern, UV-induced signs of skin aging like uneven skin tone happen to all of us - think of sun-spots on your scalp and in your hairline. 

Chronic UV exposure can contribute to the inflammation and oxidative stress that is part of the process of thinning hair / pattern hair loss, so even if you don’t worry about skin cancer, a UV-exposed scalp might be bad news for hair-density in the long run.

If you’re young - I promise, "Future You" will want you to pay attention to sun protection now.

> When one group of researchers applied hair to a mannequin and used artificial UV light, they achieved a UPF of 5 to 15. This study had outdoor workers in mind, people who have high exposure and traditionally (in the US) low compliance with hat-wearing.

> Another group of researchers used a mathematical and physical modeling system designed to estimate UV exposure on different parts of the body, and then engineered that model to estimate the effect of 2 different hair densities to and shapes to estimate protection by hairs, resulting in an estimated UPF of 30-65 for hair of different orientation and color. They believed the lab-derived estimates were too low.

> Yet another group of researchers used hairs of similar width, but different colors (brown, red, blonde, white), with artificial UV light, found a UPF of 10-40+, depending on hair color and hair density.

These two graphics combine the data. I summarized direct measurements (even though they're simulations) and added comments related to the mathematical model.

UPF provided by different hair densities, colors, length and hairstyle. Very high-density hair is left of the dotted line, average to lower density hair is to the right. Effects of hair orientation are indicated below the solid line. a 10% reduction in UPF would take UPF 20 down to 18 (or UPF 10 down to 9).

UPF based only on hair color and length. These numbers differ a little from the chart above because they taken from a single study. Length reduces UPF somewhat because it assumes your part is exposed, and the longer length weighs the hair down, further exposing the part.

Background stuff:

  • SPF and UPF are somewhat different. 
  • SPF is the way sunscreens are measured in the US, it accounts for UVB ray-protection only. UVB rays cause burning, and can contribute to skin cancer. 
    • If you have an SPF of 50, then 1/50 of the UVB rays are getting through the correct dose of sunscreen. 
    •  People usually do not apply enough sunscreen to get the full SPF on the label, this affects our perception of what that SPF actually means in terms of protection.
  • UPF is the measurement used for clothing, and includes both UVA and UVB. 
    • If “UPF 50,” is indicated, then 1/50 of the UV rays can get through. UPF is just more inclusive of all the UV rays that can damage your skin.
  • Average hair density per square cm is around 300 for people around age 30, and closer to 200 around age 70. 
    • Broken down by ethnicity (in another study of adults of a wide range of ages), 180-200 hairs per square cm in Americans of Hispanic descent, 150-180 in Americans of African descent, and 215 to 250 in Americans of European descent. Wait, why is this different than the bullet point above? Because it's a wide range of ages, and a different set of people! The more people you count, the more variation you get. 
    • Very dense hair can be 500 hairs per square cm or greater. If you have lots and lots of hair and can barely see scalp in your part, this might be you.
  • Orientation of hair matters, parallel hairs provided better protection than randomly aligned hairs in the model, possibly because they provided a cumulative effect that is lost when hairs are aligned less evenly. 
    • Prove it: If you have long-enough hair - pull it in front of your eyes facing a bright light while loose. Then pull the hairs straight and neat (and perhaps more compact unless your hair is straight). You can see less light under the latter conditions.
  • Hair color matters: Darker colors absorb more UV than lighter colors. This is true for clothing too. The molecular structure of the pigments absorb different wavelengths of light. Dark colors absorb in the UV spectrum too. 
  • Your parting (part) has no protection from hair at all! 
Real-Life Stuff:
  • If you are an outdoor worker, or spend extended time in the sun for recreation, sports, gardening, your hair is not enough to protect your scalp unless it is very thick. Even then - think of your poor part - not to mention your eyes. Hats (and sunglasses) are also great at preventing cataracts - and cataract surgery.
  • Hair-partings need protection on everybody. Any sunscreen can work, but sprays and powders can be convenient if you don’t wear hats or scarves or a UV blocking hoodie.
  • Hats that are listed as UPF 30 or UPF 50 provide good protection for scalp - and help the eyes and tops of ears.
  • Hats or head-coverings that are not UPF-listed will work better if they have: Darker colors, tighter weave (less light gets through), or more than one layer.

Birch MP, Messenger JF, Messenger AG. Hair density, hair diameter and the prevalence of female pattern hair loss. Br J Dermatol. 2001 Feb;144(2):297-304

Birnbaum MR, McLellan BN, Shapiro J, Ye K, Reid SD. Evaluation of Hair Density in Different Ethnicities in a Healthy American Population Using Quantitative Trichoscopic Analysis. Skin Appendage Disord. 2018 Oct;4(4):304-307

de Gálvez MV, Aguilera J, Bernabó JL, Sánchez-Roldán C, Herrera-Ceballos E. Human Hair as a Natural Sun Protection Agent: A Quantitative Study. Photochem Photobiol. 2015 Jul-Aug;91(4):966-70.

Parisi AV, Smith D, Schouten P, Turnbull DJ. Solar ultraviolet protection provided by human head hair. Photochem Photobiol. 2009 Jan-Feb;85(1):250-4

Religi, A.; Moccozet, L. 3D Modelling for Solar Erythemal UV Protection Provided by Human Hair. Appl. Sci. 20199, 4724.

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