Cardboard isn't the only thing. Carpets, curtains, upholstered furniture, cheap pressed wood furniture, shelves full of dead tree books, walk-in closets full of clothes, stacks of towels and bedding etc all do the same thing.

"When [hay] is baled at moistures over 20% mesophilic bacteria release heat causing temperatures to rise between 130°F and 140°F. If bacteria die and bales cool, you are in the clear, but if thermophilic bacteria take over temperatures can raise to over 175°F,”...

Most wet bales catch fire within six weeks of baling, Hartschuh says.

https://www.drovers.com/article/avoid-barn-fires-let-hay-dry...

Compost heaps also give off heat. So we know -- and this is well established fact, no need to listen to my "nutty" fucking personal anecdotes -- that rotting materials give off heat. In fact, they can give off so much heat as to start fires. So this is not an insubstantial amount of heat.

I got to see firsthand under fairly extreme conditions how this impacts comfort at home. I was sharing a single bedroom with my two teenaged sons. I had little control over what was in that bedroom. It was supposed to be a temporary situation. It was filled to the gills with furniture, carpet, blah blah blah.

Just coming home and taking all the sodas and snacks out of the cardboard box consistently dropped the temp by 5 degrees Fahrenheit. There was a thermometer in the window, so we were able to check.

After two or three incidents where we removed all the cardboard boxes from the room and the room went in short order from unbearably hot and muggy to comfortable enough where we could sleep, it became our policy to pull everything out of cardboard boxes and trash them as soon as we brought home groceries.

I currently live in SRO. It's more space than when we shared a bedroom at a relative's house and we have more control over our space here than we did there. We still remove bakery cookies from the cardboard box when we get it home and similar. We don't keep mail sitting around. All junk mail is promptly trashed.

We don't have a trash can. When we were living in a three bedroom, two bath apartment, we noticed that food sitting in the trash similarly raised the temperature in the house and also made us feel ill because me and my oldest son have a compromised immune system. It became our policy to take trash out promptly following meals. We still do that and we do not own a nasty ass trash can.

But I generally don't bother to go into all the details because I am consistently given absolute shit by the entire goddamn internet and treated like a total fucking idiot rather than having people ask for more details or express genuine curiosity or whatever. It's always "That's stupid." or "That doesn't make sense." Or "I don't believe you." or similar.

Meanwhile, people are dying in heat waves, but, meh, actually figuring out something useful isn't a priority. No. Treating some poverty stricken woman like absolute shit and like she couldn't possibly know any fucking thing of any actual use -- that our priority.

The worldn't isn't going to hell in a hand basket because there aren't any solutions. It's doing so because people who currently have money and so forth need to cater to their ego far more than they want solutions.