Small clusters and/or ions with diameter of < 2 nm are constantly present in indoor air. Number size distributions of 1-4 nm particles were measured with a nano Condensation Nucleus Counter. Measured number concentrations range from 103 to up to 106 #/cm3. E.g. emission of limonene from a peeled orange created a rapid burst of nanoparticles. Particle number concentrations of the smallest particles/clusters can exceed the number concentration of particles larger than 4 nm.
