1/ I asked for a Geiger counter for Christmas last year and got my wish. I took it on a westbound flight across the US and this .gif records the readings during take-off and climb to 30,000 feet where it recorded 25x as much radiation as at ground level.
2/ Total radiation dose measured was 8.5 microsievarts on the four hour flight. xkcd’s radiation dose chart expected even higher levels than that: xkcd.com/radiation/
3/ The part I find notable on the chart is nuclear power plant neighbors should expect to receive an additional 0.09 μSv/yr while coal power plant neighbors should expect to receive +0.30 μSv/yr. The non-nuclear power plant delivers more radiation! 
4/ It sounds incorrect but I have a personal anecdote corroborating this. As a co-op at Dow around 1999 I toured the small nuclear reactor used for fast neutron activation analysis around. A pair of stationary Geiger counters continuously monitor the lab for radiation leaks.
5/ The tour guide explained that one day the alarms rang and wouldn’t go off. A search for a leak within the building turned up nothing. The background levels were higher outside the building too. A new coal power plant had come online one country over.
6/ Radioactive metals naturally occurring in the thousands of tons of burned coal are partly expelled from the stack in fine particulate matter of uncaptured fly ash. Background radiation levels downwind of coal plants are increased three times more than nuclear power plants.
7/ The cinders remaining after coal combustion contain higher levels of radioactive metals. Concrete company turn it into ‘cinderblocks’ commonly used in construction and building foundations. Hence the higher radiation dose (per xkcd) if you live in these structures.
8/ Here’s a photo of my Geiger counter. I use the free Geiger Bot app.
