You may have wondered how safe are Garmin, Fitbit, Apple and other smartwatches that pack red or green sensor technology. After all, they are bombarding these lights against your skin nearly 24/7. Can this lead to skin damage or harm to the body?
Why do watches use red and green light sensors?
Most fitness bands and smartwatches pack red and/or green sensor technology. It is with this that they track heart rate and blood oxygen levels. This is done via a 150-year old process called photoplethysmography (PPG).
In short, an LED on the underside of a watch shines a green or red light onto the skin. This bounces back to the source which also has a photodetector which records how the light’s intensity changes. Such changes depend on the way blood perfuses through the tissue. The information is then analysed by an algorithm which converts this into data on a person’s heart rate.
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Green light is more often used than red because it has excellent signal-to-noise ratio, along with more resistance to motion artefacts. Plus there’s more existing knowledge about its use.
Red lights don’t have these characteristics. But their advantage is that our bodies do not absorb red light as well which allows them to penetrate 10x deeper into tissue than green lights. This is useful as you can use them to capture richer biometric signals.
Smartwatch heart rate technology: Is it safe? Can it cause skin damage?
The common thought might be, UV light from the sun is not good. It can cause skin cancer, premature ageing, damage of the eye and the suppression of immune system. It is for this reason we wear sunblock – in order to protect ourselves. Man-made sources of UV radiation can also cause damage.
So why would these red or green lights be any better?
Don’t reach for the sunblock to put under your watch just yet. Why? Because this is not a fair comparison. UV light is of higher frequency and energy than visible light.
Red and green lights are on the visible spectrum. This is similar to LEDs used in your kitchen lights, that lamp sitting on your end table, office lighting and LCD. None of them are harmful. Plus the power of the red/green lights on the underside of watches is very lower.
Medical devices have been using the exact same technology for years to measure SpO2. So this is a very tried and tested method.
So no, you are highly unlikely you’ll get health issues do to the LED watch lights. What you might get is light allergy. But this is extremely rare. There are people with a history of sunlight allergy and light sensitivity. Or they may have used light sensitive drugs recently which puts them at risk of such an allergy.
Not only that. Simple logic says – devices like this have been on the market for well over a decade now. If there were any significant health issues, we would have learned of them by now.
You are more likely to suffer harmful effects on your skin due to lack of air and sunlight. So it is a good idea to remove your watch or fitness band from time to time in order to allow your skin to breathe.
This is also so that you don’t get skin irritation. Which is a common problem. But skin rashes are not due to the LED lights. They come from the watch itself and the strap. Sweat and dirt can build underneath causing such issues. If you are getting rashes, check out our separate piece on this topic.
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