In the '70s when I worked with radioactive sources, we were told that the allowable annual radiation exposure (set by the NRC) was 5,000 millirems. Cecil mentioned in this column that the normal background radiation in Chicago is 2 millirems per hour, or 17,500 millirems per year. That seems a tad too high.
The average background exposure is 300 miilirems per year according to MIT.
When I went to radiation training in the late 90’s, I was told we get about 1 millirem of exposure per day - so CurtC’s cite claiming 300 millirems/year is about on par with what I’ve been told.
Also - glad to see that Cecil in the followup to the column mentioned the alpha particles. Nasty, extremely reactive things - but that is also why they don’t penetrate far. Dead skin stops em - which is why it is of little concern in the case of a smoke detector (if in fact Americium emits alpha particles).
Calculate Your Radiation Dose. Nothing special about Chicago noted there, but maybe they aren’t taking into account Cecil’s luminance.
If you want a high background level, try Ramsar in Iran:
The level in some homes there is 132 mSv which is 13 thousand millirems.
I believe this is backwards. It’s the photoelectric alarms that are more effective for smokeless, but hot and bright, fires. The ionizing alarms depend on the smoke interfering with the ionizing chamber’s conductivity and are best for smoky blazes. For this reason photoelectric alarms are good for kitchens so accidental smoke from burnt food doesn’t set them off.
Actually, Cecil was correct, but the explanation was confusing.
The photoelectric alarms depend upon light blocking smoke. Photoelectric detectors detect light. Inside a photoelectric detector is a light source (usually an LED) and a detector. When the LED goes off, the detector turns on and tries to detect the light. If there is no smoke, the detector can detect the light from the LED and the alarm doesn’t sound.
If there is smoke, it will block the light from the LED reaching the detector. In that case, the alarm will go off. Thus, you need a slow smoldering fire that creates a visible thick smoke.
Ionization detectors can detect microscopic soot particles that wouldn’t necessarily block the light. The americium creates alpha particles which ionizes the air between two electrodes. This allows a current to flow through the air between two electrodes. If there is a fast blazing fire which produces fine soot, the soot enters the smoke detector and absorbs the alpha particles. The lack of alpha particles deionizes the air between the electrodes and breaks the current.
Thus, a smoldering fire (like from a burn couch) produces the thick smoke a photoelectric detector can detect while a kitchen grease fire will produce the microscopic soot that an ionization detector can detect.