Respirable particles are a collective group of fine solid particles, aerosols, mist, smoke, dust, fibers, and fumes. The main indoor respirable particles are environmental tobacco smoke (ETS) and fibers. Some particles, such as dust, dirt, soot, or smoke are large or dark enough to be seen with the naked eye. Others are so small that they can only be detected using an electron microscope. Fine inhalable particles are defined as particles with diameters that are generally 2.5 micrometers and smaller.