The breathing of respirable quartz particles is a hazard in ceramic ware manufacture and hobby. These particles are of a size that can catch deep in the air sacks of the lungs.
Respirable Crystalline Silica (RCS) refers to particles sufficiently small to stay air-born long enough to be inhaled and go deep into the lungs where they become lodged. Extended exposures to lower concentrations or less frequent exposures to higher concentrations can cause silicosis.
The World Health Organization page at http://www.who.int/occupational_health/publications/en/oehairbornedust3.pdf says that "particles with aerodynamic diameter >50 microns do not usually remain airborne very long: they have a terminal velocity >7cm/sec. However, depending on the conditions, particles even >100 mm may become (but hardly remain) airborne. Furthermore, dust particles are frequently found with dimensions considerably <1 micron and, for these, settling due to gravity is negligible for all practical purposes."
The European Industrial Minerals Association has a website dedicated to RCS. Among others, he page
http://www.crystallinesilica.eu/content/what-respirable-crystalline-silica-rcs defines Respirable Crystalline Silica is?
Many companies produce equipment to measure dust concentrations. For example
http://www.galsonlabs.ca/resourcecenter/bulletin.php?c=20 describes measuring dust samples. It says respirable dust particles are under 10 microns in diameter, thoracic dust particles are under 25 microns, and inhalable dust particles are under 100 microns.
N95 Particulate Respirator mask
This designation is an international standard for a general purpose respirator to filter out respirable quartz particles (which cause silicosis). Use one of these when working in a area where ventilation is insufficient to remove all of the dust. Use it also in circumstances where there is temporary generation of large quantities of dust. Do not wear this as a substitute for keeping floors and working areas clean.
Why is there a health warning on the top of each box of clay?
Like any dirt, clays contain quartz. Quartz particles, if inhaled in just the right size, can block the tiny air passages in your lungs. Quartz is all around us, it makes up about 12 percent of the land surface and about 20 percent of the Earth's crust. This label is a reminder to reduce dust levels in your studio and working area. You can see specifics about hazards of any of our products by clicking links (on any manfuacturers website) to view the SDS (Safety Data Sheet). It contains references to where you can learn more about working safely.
GHS Safety Data Sheets The Globally Harmonized System (GHS) of classifying and labelling chemicals presents material hazard information in a 16-section user-friendly data sheet.
Toxicity The vast majority of materials used in ceramics are insoluble. But many still present hazards. And you can add hazards (to you and customers of your ware) by the way you use them. Still, there is a need to be realistic about it.