Sunday, 2 April 2017

Talk - Photobiology - Effects of UV Radiation on Normal Skin

Graham Harrison formerly of Photobiology Dept at St John's Institute of Dermatology, King's College London and now of the University of Nottingham comes to Café Sci to talk about Photobiology - Effects of UV Radiation on Normal Skin. @Gav Squires was there and has kindly written this guest post summarising the event, with some linkage added by NSB.

Visible light has a wavelength between 400 and 700 nanometers. Around 1000 nanometers, you're into the infrared while down at 100 nanometers, you're into the ultraviolet. The shorter the wavelength, the more energy it contains.

There are three types of ultraviolet radiation - UVA(longer wavelength), UVB and UVC(shorter wavelength). All of the UVC in sunlight is blocked by the atmosphere. Only 5% of the light that reaches the earth is ultraviolet and only 5% of that is UVB. UVA penetrates much deeper into the skin but UVB is responsible for 80% of sunburn.

In fact, UVB is at least 1000 more powerful than UVA when it comes to causing sunburn. While UVB is also responsible for the production of vitamin D, as the ozone thins there is more of it coming through the atmosphere.

We can measure the UV radiation using a radiometer. A broad band one is handheld while a more accurate spectroradiometer costs around £30,000. On the other hand, you can use a biological method and examine skin for levels of sunburn. This is done by examining the Minimum Erythema Dose (MED) - the point at which the skin starts to burn. This MED will change depending on the skin type. There are six skin types in all, ranging from Type-I (white), which has a high risk of sunburn and cancer to Type-VI (black), which has a low risk of both.

Skin Types and their reaction to UV

In all of the interactions between UV and skin, photochemistry precedes photobiology. The sunlight is absorbed by a molecule and its energy changes the molecule. This leads to a multitude of effects from tanning to sunburn to cell death to vitamin D photosynthesis. When the UV reaches the DNA, it causes photodamage. Then it binds to the DNA and can cause a mutation. It's possible to stain for the anti-bodies that are evidence of this damage. There was a time when you would have to do a biopsy to look at the scale of the damage but now it's possible to measure the excretion products in urine.

Photoageing is caused when the tissue is damaged by sun exposure. It's actually damage to the collagen in the skin and is called solar elastosis. It is thought to be a UVA effect. DNA damage is also responsible for tanning. This happens when the pigment producing cells (melanocytes) in the skin are activated. Of course, the more serious outcome is skin cancer. The UV goes into the skin and causes a mutation where the DNA is repaired erroneously. The P53 gene usually stops tumours but if this is mutated you can get abnormal cell growth(dysplasia), then immunosuppression and this can lead to cancer. However, there are a number of factors can play a part in cancer forming, including physical environment, behavioural causes, non-behavioural causes and any prevention measures taken. With skin cancer, melanomas are only around 10% of the total skin cancers but they are the ones that kill you.

The P53 Protein
As well as humans, dolphins can get sunburnt. UV radiation can also damage your eyes, it leads to cataracts. Glass protects against UVB so glasses wearers are partly protected but you can still get a tan standing in a greenhouse. Plastic meanwhile will block all UV radiation. Sand on the other hand, reflects all UV and this is why you can get sunburnt particularly badly at the beach.

UV radiation concentration is greatest at noon as the sunlight has to get through less of the atmosphere. There is also much more UV radiation in the summer. Although you do need to be careful because even on a cloudy day, there is UV damage happening to the skin. Most indoor workers get 50% of their annual UV exposure over a span of just 33 days. This usually includes their summer holiday.

Sunscreens are specifically designed to stop sunburn rather than any of the other effects of UV such as ageing. Hence they are only interested in stopping the UVB. However they are generally not used as they should be. The recommended thickness is 2mg/cm2 of skin. This would require 32g of sunscreen to cover a woman's body and 38g to cover a man's. Since you are supposed to re-apply every three hours, your bottle isn't going to last very long and it's going to get very expensive very quickly.

But what does the Sun Protection Factor(SPF) on a sunscreen actually mean? Well, if you could usually spend 20 minutes in the sun before burning then SPF6 would allow you to spend 6 times as long, 120 minutes, in the sun before you burnt. However, these tests are based on thick applications, which isn't how people use it. However, you will still get some benefit from it and even SPF2 blocks 50% of the light. SPF4 blocks 75% and SPF50 blocks 99%, which is why you can't get a higher SPF than 50. Sunscreen is also tested with a very artificial sunlight - equivalent to sunlight at the top of a mountain at the equator.

Titanium Dioxide, a popular suncreen

But the sun isn't all bad. As well as vitamin D creation, there is a feelgood factor from UV radiation.

Café Sci returns to The Vat & Fiddle on April the 10th at 8pm when Dr Marcos Alcocer will talk on Food Allergies - What Are They And Why Do We Have Them? For more information, visit the Café Sci MeetUp page:

Image Sources
P53, Tiox, Skin Types via Gav Squires.

No comments:

Post a Comment