1. My cat's cough returns when I don't keep the humidifier going.
2. My hair looks much better when I wash it in cold water.
I learned about the cat/humidifier connection when I let my humidifier dry out for a few days, thinking the spring air is moist enough that I can stop dealing with it for the season. (While not a huge hassle, it is slightly irritating to wash it between each use, so I'll be happy to have a break from it when the weather is even better.) It turned out to be fine for me, but after a couple of days, my cat started coughing a few times a day. Three days after reinstating it, she's back to normal. Great! And sigh.
I got my first inkling about the cold water/hair connection when I took a very tepid shower Saturday morning (our water heater is, evidently, not up for four showers in a row). My hair looked unusually good all weekend (I wash it every 2-3 days). So I tried cold water on purpose yesterday, and ... I'm very sorry to say it supports my hypothesis. More data will be collected over coming months, but preliminary data are very suggestive.