Friday, December 04, 2015

Possible risks in using LED light to monitor your heart rate?

I had been wearing an activity tracker on my wrist to measure the number of steps I walk daily, for over a year. It has surely given me an incentive to walk more. More recently, I upgraded myself to wear the next version, which also measures my heart rate and displays it when asked to do so. I was aware that I was exposing myself to round-the-clock LED light over a few square millimeters of my skin. After a couple of months of this exposure, I noticed that the area exposed to the LEDs had developed a tan. I was surprised; having dark brown skin, I had not expected it. I looked up information given out by the manufacturer, and it said that the energy put out by the LEDs was so small it was harmless, but I was the one who had his skin in this game, literally! I wanted to be more careful.
I do not know the exact amount of energy used; suppose it is 20 milliwatts and that it is spread over two square millimeters. The energy density would then be about 10 milliwatts per square millimeter. Compare it with 140 Watts of Sunlight spread over a square foot; that is about 1.56 milliwatts/square millimeter. Besides that, I am unlikely to sit in the Sun giving me 140 Watts/square foot of radiation round-the-clock! Governments have advised people to be careful in sunbathing, to reduce the incidence of skin cancer. Visit The Surgeon General’s Call to Action to Prevent Skin Cancer. In many countries, where the weather encourages frequent sunbathing, deaths due to skin cancer exceed the number of traffic deaths.
After thinking all this out, I read the documentation again and found I could turn heart-rate monitoring on and off as I liked. Now, I keep it switched off most of the day and turn it on only during exercise sessions. I have also given up wearing the gadget during sleeping hours. You think I am paranoid? Give me a few reliable studies by credible and independent medical researchers, concluding there is no risk. I will forget my worries!
Subject to standard ethical safeguards, it may be worth doing a small-scale animal experiment exposing rats to the same HR monitor that I have been exposed to. The rats might be smarter and bite the strap off before it is too late! The experimenters might have to take some precautions.
It may be necessary to have the rats’ exposed skin examined by pathology labs to identify any possible risks from the exposure.
I would also suggest that students interested in doing the experiment to send their proposals to the manufacturers of the devices asking for a grant of 60 devices for use in the experiment. You can assure them that you will acknowledge their contributions when you publish your findings. The results might be interesting!
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5 comments:

Srinivasan Ramani said...

There is a follow-up item in regard to this post - a photograph of skin damage on my wrist caused by the activity tracker. I could not include that photograph in this comment and hence made a new post. Visit Possible risks in using LED light to monitor your heart rate Post No 2

Srinivasan Ramani said...

It is almost a year since my first post on possible risks in using fitness trackers that do LED monitoring of your heart rate (HR). Now, I control the the amount of time the LED's are on by switching off HR monitoring when I am not taking a walk. Despite this the black spot on my wrist has been getting darker. I have a couple of suggestions:

1) Use a moistening cream on the wrist. It controls the drying up of the skin exposed to LED light.

2) The makers of fitness trackers should modify their software to have the LED's on only when you bring the HR display on. Rest of the time, the LED's could be off. This will reduce risks.

Jonathan Seyfert said...

There are a couple concerns I have with your post.

Firstly, what causes skin tanning and cancer is UV light. There is some limited evidence that visible light may cause indirect DNA damage, similar to that caused by UVA exposure, especially in the upper portion of the visible light spectrum, but there is still little information on this. The watches certainly do not have any light produced in or near the UV spectrum. Green light is used for heart rate monitors.

Secondly, the assumptions for light power density used seem a bit off. The power output perhaps not. A typical low power LED may run about 20 mA forward current, with about 2.2 volt forward voltage. This is 44 mW. However, traditional green LEDS (producing green light directly as opposed to down-converted blue LEDs) are about 34% efficient. So there's about 15 mW of light. This isn't far off of your assumption. However, based on observation from my watch, the LEDs do not output at full power most of the time. They have variable light intensity, and ramp the power up or down as needed to get a proper heart rate signal. Additionally the LEDs are flashed, not steady, so the average power output is much, much lower.

We can do a quick calculation to confirm this. My Garmin Instinct can run 24/7 heart rate monitoring, with a battery life of about 2 weeks if GPS is not used, with no solar power battery life extending, and limited backlight or smart features are used. The aforementioned LED at 44 mW, running 24/7, for 2 weeks, would use 2.2 Wh of energy. A 2.2 Wh battery would be approximately as large as the watch itself, leaving no room for any electronics! Certainly the LED cannot be using anywhere near this much power! The size is not listed but a Google search suggests approximately 200 mAh for battery size. This seems reasonable. If the watch used 100% of the power in a 200 mAh battery for the heart rate monitor and no power for the rest of the watch (display/backlight/bluetooth) then the maximum average power draw of the LED would be 0.6 mW. This is two orders magnitude less than my original guess with a "typical" LED. It would have to be lower still, as the rest of the watch uses power, but now we're in the ballpark. So we'll say the LED draws 0.6 mW on average, and at 34% efficiency then the light output is about 0.2 mW.

Next we'll address the area. 2 square mm is way too small. The opening for the lights (there are two lights with two openings) on my Garmin watch measure 2x5 mm, or 10 square mm. But even though there are two lights, we already established the total average light output was 0.2 mW, so this has to be split among both openings. So 0.2 mW over 20 square mm is 0.01 mW/mm^2.

Your value for sunlight of 1.56 mW/mm^2 is also too high, average peak irradiance is about 1000 W/m^2, or 1 mW/mm^2, and this is peak levels, which is at noon, and lower through the rest of the day. So with our much more detailed estimate, the light output reaching our skin from a heart rate monitor is approximately 1/100th that of sunlight, not multiple times as much! It also contains no UV light, which is very much the predominate factor in skin tanning and skin damage from sunlight. Note for example that UV light is only approximately 3% of the light coming from the sun (3% of 1000 W/m^2 or 3% of 1 mW/mm^2), and sunscreen only blocks UV and not visible light, yet proper application and usage of sunscreen drastically cuts the amount of tanning and sunburn we get.

In summary, while visible light may cause SOME damage, it's the UV light that causes the VAST majority of damage, and this is only 3% of sunlight causing orders of magnitude more damage than the 42% of visible light (the remaining 55% is infrared). And the average amount of light from a heart rate monitor is only 0.01 mW/mm^2, or approximately 1/100th the total solar irradiance at noon on a clear day.

Jonathan Seyfert said...

Some final thoughts that didn't fit on the first comment:

While makers of fitness trackers could modify the LEDs to be on only when you bring the HD display on, this defeats the purpose of 24/7 HR monitoring. 24/7 heart rate monitoring is done for the monitoring, not the display! By having 24/7 HR monitoring, stress levels through the day can be measured, resting heart rate tracked, calories can be determined to a much greater level of accuracy, etc. Depending on the watch, abnormally low or high heart rates can trigger an alert, which may allow a person to seek medical attention before a heart condition causes them to suffer irreversible brain damage or death. All that said, my Garmin does have that feature, sort of. If I turn on battery saver, this turns off the HR monitor (as well as bluetooth, making it not a smart watch anymore). However, even if battery saver is on, if I start an activity such as walking, biking, or running, it turns the heart rate monitor on for the duration of that activity, turning it back off when the activity is finished.

I suspect you may have some sort of contact or allergic dermatitis. I note in your update post with the picture of your wrist, you point out the dark area where the sensor sits, but I noticed a small dark line where the band itself sits. You might seek out the opinion of a dermatologist.

P.S. If it is some sort of contact dermatitis, putting lotion under the watch is a bad idea! No sunscreens, bug sprays, lotions, or other chemicals should be applied to the skin under any watch, not specifically one with heart rate monitoring, as all of these will greatly increase the chances of contact dermatitis.

Srinivasan Ramani said...

I had replaced my tracker two years or so after purchasing it. The replacement is better in many ways. For one thing, the blackened spot on my wrist gradually faded away. Moral: watch for damage to the skin in the first few weeks after purchase. If you discover any during the return period, return the device!

Secondly, There is no evidence in my case to suspect skin allergy, unless there is allergy to certain brands!

Srinivasan Ramani