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The dangers of radium luminous paint were recognised in about 1930, and were mainly due to the ladies doing the painting using their mouths to 'point' the brushes. Mostly they had been recruited from the ceramic painting industry where this was normal practice. There is an interesting analysis and report showing the deaths attributed to radium paint use are negligable after 1930 when this was known, despite widespread use in WW2.

Present dangers are mostly if the paint is scraped away dry to produce particles and dust containing radium which could then be ingested or inhaled or produce contamination locally.

There is an article I found whilst looking at the various internet articles by a physics teacher on how to make a good alpha and gamma source by dissolving the paint in acetone to concentrate it in a small container. At least this method will not produce dangerous dust.
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The reduction of luminosity is mainly degredation of the zinc sulphide crystals with radiation damage. This effect of crystal lattice deformation is used as a means of measuring low radiation levels fairly accurately with the phenomenon of thermoluminescence dosimetry.
Some crystaline substances (calcium flouride and lithium flouride and many others) will produce visible light when heated after radiation exposure as the dislodged crytaline lattice is re-formed. This can be measured with a photomultiplier tube and is corellated with the radiation exposure. I was involved in the calibration of exactly this system method for use in the applied physics R and D dept for the British Polaris nuclear submarine programme in the late 1960's.

I used to use accurately calibrated sources kept in a large walk-in safe, which also contained the Cobalt60 'bombs' for X-ray of the steel hull after welding.

One day the radiological 'elf and safety' man, a Mr. Winstanley, ( who was a brilliant raconteur who could keep you amused with stories for hours) said: "go into the safe and this time don't switch on the light, take out what you see and measure it .. Oh! and by the way use the long tongs". Which I did.

The object in there which he was referring to was very obvious: a small round disk about 2" diameter and glowing _very_ brightly indeed: you could read a newspaper next to it in the dark easily. I took it out into the lab and switched on the radiation monitor about a couple of feet away from it. the needle banged over hard. I should mention the meter goes onto the _highest_ reading scale first, and is switched down about six ranges to lowest where you can then read backgound radiation. I put the object back into the safe fairly quickly holding it away from me as far as posisble - a good yard with the tongs. Mr. Winstanly was laughing. "A bit hot isn't it?" he said. Yes, I replied, "what is it and where did it come from?" Winstanley said: "we don't know where it came from originally, but it was found hanging off the back of the saddle of someone's bicycle !! ...it set off the alarms at Windscale as he approached the gate dozens of yards away".

I hate to think of the effect upon the bicycle owner's wedding tackle.
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Here is a good appraisal of the reduction in luminosity and general facts concerning luminous paint:

from:-

http://www.orau.org/ptp/collection/radioluminescent/radioluminescentinfo.htm

COPY:-

The Deterioration of the Paint's Luminosity

Over time, the intensity of the glow from the paint will decrease because of the damage caused to the ZnS crystals by the alpha particles. Since radium-226 has a 1600 year half life, its decay is not a significant factor. Robley Evans once told me that he could estimate how old radioluminescent paint was by examining the size of the ZnS crystals under a microscope. As I understand it, the crystals actually begin to fragment because of the damage caused by the alpha particles. The damage presumably prevents electrons in the conduction band from reaching the activator sites (e.g., copper, silver) where the light would be emitted. The higher the activity of the radium-226 (and the brighter the luminescence) the faster the deterioration. One early report by Charles Viol stated that the reduction in the luminescence was due, at least in part, to a radiation-induced discoloration of the zinc sulfide crystals. He also indicated that the damage could be reversed by heating the paint. This explanation is plausible and might provide a partial explanation for the decreased luminosity of old paint, but I have my doubts. One of the best kept trade secrets of the various manufacturers was how they minimized the inevitable decrease in the luminosity of their paint. A couple of the factors that would affect this decrease in luminosity would be the amount of the radium, and the size and concentration of the zinc sulfide crystals. I suspect that the type and concentration of the activator sites would also play a role.

In the 1920s, "mesothorium" was sometimes added to the paint - mesothorium is the historical name given to radium-228 (5.8 year half-life). Ra-228 is a beta emitter, but over time it decays into thorium-228 (1.9 year half-life), an alpha emitter. The result is that the intensity of the light from the paint will increase over the first five years or so due to the in-growth of the Th-228. The practice of adding mesothorium ended by 1930. Any mesothorium that might have been used will no longer be present, it has long since decayed away.
ENDS.

Douglas Denny.
Chichester. England.
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