When is it "ok" for soldiers to deblouse while in garrison?

Refer to 1SG Hicks' comment. He pretty much said it. And yes, I've taken the heat for my soldiers' welfare on a number of occasions.

I would love to clear this one up for you. My PMOS is preventive med specialist and my SMOS is medic. I have been a medic for 12 years so this topic is dear to me.

Bottom line up front: According to ATP 4-25.12, ch.2-86, "When the situation requires and the tactical situation permits, commanders and leaders must modify the wear of the uniform to ensure that soldiers' skin is covered while in sun and uniforms are worn loose at neck, wrists, and lower legs (unblouse pants based on the heat category).
This is my favorite reference for field related preventive med because it applies to all levels of command. Others have discussed the importance of keeping the skin covered while in the SUN so I won't harp on that, but it is acceptable from a preventive med perspective to remove tops when NOT in the direct sun, i.e. while taking breaks, which can be as often as 10 minutes every 30 minutes of work (at the highest heat cat). The exception to unblousing or removing tops is when there is a moderate to severe arthropod risk.
At the end of the day, unblousing and removing uniforms remains the lowest level of risk management and medical personnel are much more concerned about the higher line items such as work-rest cycle and hydration. I highly suggest reading up on this section of the ATP I referenced for heat injury mitigation policy recommendations as it is a current priority of the Army, due to recent and past heat-related deaths.

Personally I don't burn easily. However, that changed in 110+ degree heat at Fort Sill. I burnt both lower arms almost purple wearing just a T shirt and unblosed trousers while on a four day exercise. I grew up on a farm in Oklahoma and had had neck and shoulder sunburns before with one other time of light burns on my lower arms.
As the maintenance and repair supervisor I didn't force my guys to keep their blouses on, but they did have to keep t shirts on *and* be aware of sunburns even after hours. I've spent many summers here working outdoors on the job, the farm, etc. Everyone who has worked for me has had the common sense to avoid a sunburn. I'm often the darkest skinned Caucasian in the group and I usually get burnt the most because I don't use sunscreen.
In the sun, you get a choice: sunscreen or cover up. Inside the ship (yes I was Navy and yes I did time at Fort Sill), just keep it decent unless there's hot machinery or chemicals to look out for.

I would say once we reach heatcat temperature zones to authorize deblousing of the boots and unbuttoning of the sleeves to allow for air flow but to keep the actual blouse/coat on to protect from the sun. Additionally if you're on an outside work detail in such temperatures the boonie should be authorized to increase sun protection around the neck and face area. If we can do it overseas in the name of safety, we can do it at home, but garrison is a different beast of "hurr durr I'm in charge", sometimes.

Everything is dependent on your leadership though, it doesn't hurt to suggest it if your argument is solid. I've seen quite a few reasonable decisions by SNCO's and officers if given a good reason as to why.

Another thing to consider for outdoor work details is someone should be monitoring rotations. You can't expect someone to work consistently in 100F+ without diminishing returns and the increased opportunity for injury.

Reg or not, I would say a lot of this is in the territory of taking the appropriate actions in the absence of orders for the safety and continued productivity of your Soldiers.

Yikes. As a former UAS PL I can attest to the lack of shade on the flight line. Unfortunately structures, tents, and trees don't work great near runways. The simple, though somewhat questionable answer, is for platoon leadership to take a little risk and let the soldiers de-blouse for a bit when the LT or WO is the ranking person. I'm decently versed in safety regs and I don't think there is one directly supporting de-blousing in any condition. If someone finds it I want to know too. I'm big on making life better for soldiers using regs to back me up. It's hard to argue with both regs and "this will make soldiers lives better."

However I found this article which might help you make a reasoned argument:

https://safety.army.mil/MEDIA/Risk-Management-Magazine/ArtMID/7428/ArticleID/6922/Plan-for-the-Heat

From another article here:
https://phc.amedd.army.mil/topics/discond/hipss/Pages/Heat-Related-Illness-Prevention.aspx

"Leaders: use risk management guidelines when planning for training in the heat"

And that is the correct Army answer short of a regulation.

Also I suggest that you encourage anyone that is giving guidance on how soldiers should be dressed while working in the heat to go help the crew chief from fueling to launch or ask them to help with the runway emplacement particularly pounding stakes for the arresting gear. The risk of sunburn is higher when de-bloused so I suggest de-blousing while out of the sun, such as in hot tent, then re-blousing when going into the sun with cuffs unbuttoned and the top partially unzipped.

Soldiers do not "decide" when to deblouse. That is up to command and the NCOes under that command. Operations is well aware of the heat index expected for the day and will issue orders appropriately when heat reaches cat 3, cat 4 and above, as well as what remedies and modifications that are to be taken at that time.

Usually the way it runs is a detail is under the supervision of an NCO who decides blouse on or off. Examples being sterilizing poop barrels, filling sandbags, digging entrenchments. A lot of times this is over ridden by officers, some crusty NCOes and some seriously out of touch upper echelon officers.

I generally fell back on the thing of making the call for my detail, detachment, section etc., when it came down to it and I felt the situation warranted it. If an NCO or Officer appointed over me had an issue with my order to my troops they could take it up with me. Some did and I always defended my decision and only got flack a few times over it. I stand by that call. As NCOes it is our job to lead, not stand by.

It’s ultimately up to the commander whether soldiers can deblouse. If heat cat 3 per the dry bulb most places have unbutton of sleeves and unblousing of trousers to allow cooling, but protect from sun light. Also many schools have immersion tanks for putting the arms into for a quick decrease in body temp.

If the above aren’t being done by leadership it’s just sad. I’m sure once they have a couple heat casualties or someone dies from heat stroke suddenly things will change. Unfortunately that seems to be how things go.

As a FNG infantry solder in Vietnam I was sent out to join up with my unit at an LZ. Those 2 days on the LZ I was filling sandbags all day. It was about 115 degrees and humid so I took my OD jungle shirt and t-shirt off and continued to fill sand bags. The next morning I had water blisters all over my back and was in a lot of pain. I asked Top if I could stay back in the rear for a couple of days and was told if I did that I would be given an Article-15 or I could put my back pack on and put my M-60 on my shoulder and move out with the rest of the guys and would not be given an Article-15. So I asked one of the guys to help me put on my 75-100 lb back pack and as soon as I put it on all the water blisters started popping and it felt like cold water on my back. Long story short 55 years later no matter how hot it is I leave my shirt on.

I was a 68w for a little over 18years before getting medically retired. The 1st thing is that you will actually cool down better with your top on and sleeves down the dampness of the sweat will help cool you with any type of breeze as well as help prevent you getting sunburned. The biggest problem i see in your question and statement is that there is no heat cat work cycle protocols in place. Every post has a heat cat work/rest cycle based on the heat category and relevant ambient temperature

Hmm and here I thought they were talking about "boot blousing"
Back in the day