In my experience, how you're wearing the watch is more often the cause of the irritation rather than the material itself. I have experienced some irritation over the years, but usually due to excess heat and humidity, or wearing the strap too tightly. The material itself is used precisely because it is highly unlikely to cause an allergic reaction.

Dec

I had to stop my optical HR after peeling skin developed under the sensor. I was wearing it 24/7 only taking off to shower. I'm fit as a fiddle never any a allergy.

I though having a light against my skin for weeks on end might have some a adverse effect.

One benefit from turning off the optical sensor and also the Pulse OX was battery life improved. I do miss the extra stats that I don't receive now but I'm hesitant to turn the sensors back on 24/7

AllanOlesen67, Now that you characterise it like that, it is exactly what happens to me, it is the under the strap loops that I get a rash. So clearly it is not allergy to silicone, I think it only occurs in hot weather when the strap is tight. So it seems to me it is about allowing the skin to dry out and breath. Nevertheless it is on and off and I now wear the titanium band and have no issues.

I had been wearing that band rather tight so that those sensors would have good contact with the top of my wrist so that might also be causing some of the problem with the skin irritation.  I have stopped wearing the band for a few days to let my skin heal and it's healing quite well.  I'm going to try some lotion with vitamin D and not wear the band as tight as before and see what happens.  If the irritation occurs as before I will change bands and likely go to the nylon band as my next choice.  I have never had any irritation from stainless steel bands worn 24/7 so I know that will likely be my final solution if the nylon doesn't work out. 

I have exactly the same problem, and as you point out it is the strap loops that cause the issue. It seems ok in cold weather and if the strap is worn loosely. But in the heat I get the red shiny peeling skin just under the strap loops. I think it is because they are proud of the strap they exert occlusive effect on the skin. This is not allegy. My solution is to wear a titanium strap, although I would like to wear outrageous colours occasionally ... maybe in winter.

So, I've been wearing the nylon/leather trim band I bought on Amazon 24/7 for well over a week and that band is absolutely wonderful.  No discomfort, skin marks, or anything else.  It's very comfortable as it seems to breathe.  There's never any dampness under the band and I don't even notice that it's on my wrist as it's relatively light.  I would say I'm wearing it tight enough that it doesn't flop around on my wrist.  I have only taken off a couple of times to rub on a little bit of skin protection ointment and then put the watch right back on after I updated it and charged it.  No more problems here.  I sleep with it on and it doesn't bother me in the least.  I can toss that silicone band in the garbage as I'll never be wearing that strap again.  I definitely recommend the Yooside/Olytop black nylon band for anyone with skin sensitivity problems.  Thank heavens for Amazon's after-market products.  This Fenix 6X Pro is the first high-end fitness watch I've had and I'm really liking it.  A far cry from the Vivofit 1 I wore for a couple of years.  I love the battery life and the ability to play music.

Whats the sizing like ? I ask as I've had a few of these straps where they've just added adapters onto an existing strap and they are far too big. Whats the minimun size of wrist it wil fit?

The minimum size is 6.3" around.  My wrists aren't all that large and there are four holes left (about half an inch) for me to tighten but I'm sure I won't need them.  It was the first strap I bought and it's perfect for me.  Go on Amazon and enter this search term to see the item for yourself: 

Fenix 5X/5X Plus Quick Fit Watch Band,YOOSIDE 26mm NOTA Nylon Durable Sweatproof Watch Band Strap for Garmin Fenix 5X/5X Pus/Fenix 3/3 HR/D2/Descent Mk1, (S-Black)

I did experience it occasionally, but only in really hot weather.  When travelling I never took the watch off, even when showering and at some stage attributed the irritation to the possibility that a combination of trapped moisture and soap may be the cause and stopped wearing it in the shower.  The irritation went away but would still occasionally manifest itself and swopped wrists until healed and then back to the left wrist.  Even the other wrist became irritated.  I then started following a regimen of washing the watch every day before hopping in the shower, regardless of whether I'd had a strenuous activity or not and after an activity would also rinse it.  Problem went away entirely.  A friend experienced the same recently and we isolated it to his magnesium intake which he has reduced significantly and the problem went away.  In retrospect I was taking magnesium supplements at the time I had the issue as well and did stop around the time the watch stopped irritating the skin.  So that may have been a contributing factor.  Still stick to my cleaning routine, and have had no issues for over 6 months.