Let's not ignore the elephant in the room. A sedentary lifestyle with minimal exercise has become the norm.
(I would almost expect, that the study authors know of something like that, and would have mentioned it, if it were the case.)
The story of willowfine is roughly as well known as penicillin. The point is that in 1910-1920 people didn't carry NSAIDs in their purse and pop 400 mg when they felt a headache coming on or their back hurt. That's a big difference.
It's also worth noting that right at that point, the supply of aspirin was very limited due to phenol shortages caused by the war.
Human body temperature varies naturally by about 5 degrees F. This study is talking about a .4 degree change. At the very least, your body can definitely handle it. This is purely an interesting phenomenon that may not even be real.
Aspirin is very well-studied and definitely known to be very beneficial for your lifespan and quality of life. You should absolutely not throw away all that knowledge because it may be tangentially related to arbitrary theorizing (about inflammation) on a strange result on sketchy data from uncontrolled groups with unknowable biases. Medicine is proven to be good for you. Even if it does have unknown negative effects -even if this is an effect that we don't know about, that may not even be bad- you should not disregard medical wisdom for the sake of what you don't know.
Relatively speaking, it also has a fair low LD50. For a person around 150lb, 40 standard 325mg pills can be fatal. In fact you'll almost never, outside of hospital supply, find the 81mg chewable version in quantities higher than 36 in case children get a hold and think they're candy. Even then, a 2 year old downing a whole bottle would still be close to an LD50.
All in all, despite being extremely common and cheap to the point of nearly free ($10 for a year's daily supply) it is actually less safe than other NSAID class drugs. But then none of this stuff, strictly speaking, is "safe" if it is used outside of it's normal use parameters.
Given that the people involved are in their 20s I would guess that obesity plays less of a role than you'd expect.
[1]: https://en.wikipedia.org/wiki/Obesity_in_the_United_States#/...
The people being studied were in their 20s- obesity has increased 4-5x among children since 1980, but it's still only 20-25% for people in their 20s. Since then we've gotten gym class and advil- they had barely gotten aspirin.
Selection bias probably plays a very large part. They may have been able to measure temperatures but I doubt they were rigorous about taking representative cross-sections in 1910. Additionally the bias towards obesity has roughly switched- wealthier people were more likely to be obese in the 1900s, and now poorer people are more likely to be obese.
That said, muscle mass correlates almost directly with energy consumption. Muscles literally require energy to stay limp. Rigor mortis is caused by the supply of energy being cut off. The majority of the body's energy requirement is spent just keeping your muscles in working order- not even moving.
I think there were a lot of these little things that mean that yes, our 'sedentary' grandparents and great-grandparents were a lot more active just because of how life was.
If you're sitting on your ass all day and then going out to drink a gallon of HFCS-infused coke (free refills) and a low-budget meal that could satisfy a starving family, of course you're gonna get to this point.
My personal thought with the US is "why does everything have to be so big?"
Huh? fitness industry is booming! Gym attendance is near all time highs! More and more people are eating healthier and switch to healthier and more active lifestyle.
The rates of intentional exercise - going to a gym and working out - are certainly on the rise, but the actual rate of exercise is more lower than it was a hundred years ago. The job market has switched in large part to sedentary jobs from active jobs. And diets are much less healthy today than they were then because the only diets available (to anyone other than the rich) were relatively healthy. It’s hard to become overweight without the presence of junk food. Not impossible, certainly, but hard.
That aside, the brain burns a significant percentage of calories burned. Perhaps on whole we are thinking less?
Whatever happen to HN being a space where a legit "what if?" was explored, not knocked?
CO2 levels for example can negatively impact cognitive function [1], perhaps this could have some impact on energy expenditure in the brain which might reduce body temperature some. Bodies will absolutely adjust to temperature changes as well (to some extent). Plausible but probably not a large percentage of a body temperature.
I could see people being exposed to higher CO2 levels though due to the fact there are more and more indoor jobs, and less due to climate change. Even with HVAC systems, a quick Google query makes it seem that CO2 levels are generally higher in offices than outside [2] [3] [4] [5].
And while we're on that brain line of thinking... perhaps the increased consumption of television and mindless scrolling of social media feeds also results in far lower cognitive engagement which also decreases energy expenditure shrugs. Also worth looking at I suspect.
For me I immediately thought of the thermic effect of food. Legumes, many spices, nuts, seeds etc all tend to have higher thermic effects. Perhaps eating a highly refined carbohydrate diet has caused a lowering in body temerapture by having lower thermic effects (I don't know, this was just my first mental leap). I suspect there is something here but probably not a major percentage of the difference.
[1] = https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037
[2] - https://www.businessinsider.com/office-air-co2-levels-making...
[3] - https://www.ncbi.nlm.nih.gov/pubmed/26273786
[4] - https://www.planteriagroup.com/blog/co2-levels-in-offices-ar...
[5] - https://www.washingtonpost.com/news/energy-environment/wp/20...
More importantly, you can't infer anything about the CO2 levels when they were being measured from the global CO2. Big buildings would have had very high CO2 levels since there's no central air. Small buildings would have much lower CO2 than any current buildings when the windows were open, and much higher otherwise. Oil or gas lamps would raise the CO2 levels a ton. Proximity to coal furnaces would have had a huge impact. Fireplaces too.
Bottom line: global CO2 levels have virtually nothing to do with how much CO2 people are exposed to.
Some of these alternatives, like temporal scan thermometers vary dramatically.