MacBook Air at 5700 RPM fan speed during summer? it is unusually hot today (37 Celsius, about 100 Fahrenheit) and my MBA mid 13 fan is going crazy. The CPU temp is at 47.5 when almost idle but my fan blows with 5703 RPM (pretty much the maximum). This data is taken from iStats and the fan symbol is literally blinking red.

Is there anything I need to worry about or is everything cool as long as the CPU is still cool? I understand that with an outside temperature almost as hot as the CPU temp, the fan needs to blow a bit more, but I am afraid the fan blasting at 100% could have some unfortunate outcome. 

Going into my basement, it goes down to about 4000 RPM, so the fan itself does seem to react to the temperature as it is supposed to do.

2nd question: should I decrease my MacBook use (or maybe cap the max CPU load) just to be safe or is Apple's temperature management smart enough to turn down when it's getting dangerous?
47.5°C is normal operating temperature and you have nothing to worry about. When your computer reaches around 100-110°C (~230°F), it will take cautionary actions. Your computer will automatically cap CPU load depending on the temperature. 

It also depends how long you are using your computer at these hot temperatures. If you are doing CPU intensive tasks on your laptop (3D gaming, rendering, etc.) for extended periods of time, you may want to look at manually controlling the fans. iStat has built in fan control settings but I also use Macs Fan Control. In this you can increase the Fan speed and set automatic fan speeds. Do note that you shouldn't always run your computer with max fan speeds because that will slowly decay the life and effectiveness of your fans (you usually don't need to worry about this).

If you're like me and use your computer as a workhorse, you may want to consider purchasing an external fan (only get one if you are really stressing your computer on a daily basis).

Usually, you don't need to do much but you can always help and care for your CPU by keeping it cool (which will extend it's life). If your computer is constantly getting very hot (~90°C) you might wan to start taking measures. 40°C is usually nothing to worry about.

Also, the only hot air outtake is at the bottom of the MacBook so you want to make sure those aren't covered up, if you use your MacBook on your bed, maybe even your lap, try seeing if you can get a desk to help ventilate the laptop. 

The laptops temperature is definitely influenced by the temperature in the room. Because you say it cools down when going to the basement, it sounds like a ventilation thing.