GuyW

Dr Mike

there is supposed to be a counterdiffusion issue with breathing a slowly diffusing has (e.g. Air) whilst in a quickly diffusing atmosphere (e.g. Helium). From Bennet and Elliot's book:

COUNTERDIFFUSION: RELATION TO DECOMPRESSION
‘Counterdiffusion’ is the term used to describe the
physiologic effects of an individual’s being exposed to
two inert gases of very different diffusion properties. It
relates to decompression in two ways. First, the
symptoms of the two ‘gas lesion diseases’ – DCS and
counterdiffusion sickness – are similar and the etiology
is apparently the same – bubbles in the skin or gas
emboli elsewhere. Second, counterdiffusion situations
can complicate decompression or can aid it, and
counterdiffusion lesions can happen with or without
pressure changes. There are two categories of inert gas
counterdiffusion as defined by Lambertsen, a ‘superficial’
condition and a ‘deep tissue’ condition (Lambertsen
1989).
Superficial counterdiffusion
Superficial counterdiffusion occurs through body surfaces
when a heavy or slowly diffusing gas is breathed (e.g.
N2 or Ne) and the external environment is a lighter,
more rapidly diffusing gas (e.g. He). A net inward flux
of the lighter gas into the skin causes subcutaneous
supersaturation and extravascular bubble formation,
which can occur without pressure change. In human
exposures at elevated pressure, intense itching was
accompanied by hard, raised, bloodless lesions and severe
vestibular dysfunction (Harvey & Lambertsen 1978,
Lambertsen et al 1977).
An opposite effect occurs when the switch is from
helium to nitrogen as the background gas. If a diver
breathes helium–oxygen at depth and changes to air
during decompression, helium leaves his tissues faster
than nitrogen enters and the resulting undersaturation
appears to allow accelerated decompression, does not
appear to cause counterdiffusion problems, and is a
well-established practice for most deep helium–oxygen
bounce diving (Hamilton 1976).

If you are in an Argon or Air atmosphere your are ok. Generally this is not a big issue as people generally don't use helium in their drysuits. You would freeze your nuts off.

Guy