NIOSH: Exposure limits are uncertain for nano particles
Nanotechnology is a broad name given to a wide range of technologies and materials that create, manipulate, or use particles that have one thing in common - their size. Nanotechnology (or nanoscience) involves materials that are extremely small and have dimensions roughly between 1 and 100 nanometres (nm). A nanometre is 1 billionth of a meter. To give you an idea of the scale of nanoparticles:
While the exact definition of nanotechnology may vary, most research and studies have concentrated on particles with a dimension of less than 100 nm.