Reference the discussion of if it is dangerous to put your fingers
in a Ultrasound tank....
The real answer is "it depends"
The answer depends on the relative power output of the ultrasonic
transducer, the distance you are from it, the viscosity of the
liquid (Note I am not assuming water only), The temperature and the
Think Radioactivity (In this case gamma Radiation) same idea... Air
Line pilots are regularly exposed to elevated and easily measured
levels, is it dangerous, we don't think so. A person working at a
factory using a Gamma irradiator will be condemned to death in
moments by the radiation levels. In both cases same radiation, but
the variables are different.
coming back to Ultrasonic cleaners, to be honest I doubt if consumer
type equipment would be dangerous, even the Jeweler type bench top
is probably of such low power that the risk is negligible. The larger
industrial high power units that for example a casting operation
might invest in to de-invest castings in large batches, dangerous.
I would say ask the manufacturer and / or verify the documentation
you received with the unit, but in today's heavily layered up north
American environment, you can be sure that the warnings will be more
to protect a Manufacturer from a person who develops arthritis in
their 60's claiming that the ultrasound caused it and preventing the
resulting lawsuit, rather than what the real risk is. (For a laugh
the most ridiculous warning label I have read about was a "Dangerous
if swallowed" on a fishing lure with a large 3 tine barbed hook).