This video claims to show mercury giving off a steady stream of toxic vapor when viewed with a UV light.
http://www.youtube.com/watch?v=JABbofwD3MI
There is also a Reddit discussion about this video. The visible vapor could easily be something else besides mercury. Does mercury really give off toxic vapor?
As already mentioned by w00t, it is plausible that the video is actually showing mercury vapor. Mercury vapor is also used in specialized lamps (Mercury-vapor lamp), the mercury spectra are well known. Likely the green background you see is a TLC-plate coated with a fluorescent dye and you're seeing the shadow of the mercury vapor on that plate.
Whether the rate of vapor you see is realistic I cannot say, it could easily be faked by increasing the temperature and thereby the evaporation rate. Also, judging the amount from the visual appearance of the vapor could be very deceptive, the far better way would be to measure the mass of the drop and calculate how much mercury evaporated.
Luckily, someone has performed experiments on the evaporation of mercury drops. The rates they measured for a small and a larger drop were 6 and 7 µg/h.
The author also extrapolates how much mercury it would take to achieve vapor concentrations that reach the OSHA limit.
How much mercury would it take, under the assumptions of the present study, for the evaporative concentration to reach the OSHA limit? This amount would depend on the size of the drops, as well as the overall time period. Assuming drops of the smaller size studied, with approximate diameter 3 mm, it would take 2000 such drops, for a combined mass of 400 g over the full time period. If we assume drops of the larger size studied, with approximate diameter 7 mm, it would take 500 such drops for a combined mass of 1000 g. These are large amounts of mercury. These crude estimates are not inconsistent with direct observation of a large spill.
Elemental mercury is toxic, but the danger depends heavily on the amount of mercury in the atmosphere. The amount present in common household items are usually very small, so they usually don't represent an acute danger. You should still clean it up properly, but there is no need to panic over small amounts of mercury.
Mercury is toxic in its elemental liquid and vapour forms and in its compounds, though to different degrees. Hence the advice from the US EPA on how to deal with spills.
Like all liquids, mercury will continue to evaporate until it reaches its saturated vapour pressure or it is completely gaseous.
While the liquid and vapour are toxic, elemental mercury poisoning is rarely fatal, though there was a reported case of a dental assistant who died after continued exposure. According to this review, after a high acute exposure to mercury vapour, symptoms such as cough, dyspnoea, chest tightness, lethargy, restlessness, fever and signs of pneumonitis can develop.
As with most toxins, the higher the dose, the more serious the response. Minor cases, such as a broken thermometer, are much less serious, but advice (such as that from the EPA) on to deal with the spill including opening a window, and what not to do including using a vacuum cleaner, should be followed.
I'd just like to point out that what you see in the video under UV is not the vapor itself but the shadow of the vapor under UV. The light is almost parallel with the plane of the dish and at some points in the video you can see the vapor coming off one of the sides of the dish but with a gap between the dish and the shadow of the dish.
As you can see in this image (from here), a mercury lamp does have several peaks around 400nm, which is UV spectrum. So pure mercury vapor would absorb UV and hence cast a shadow.
(source: cbu.edu)
Therefore I think that what you're seeing in the video is indeed mercury vapor, which is indeed toxic.
