The technics established for the study of Radon and Thoron daughters were applied to the determination of y activity on particles of a radius below 2.10V cm at ground level. Long time samplings were used and the collections were measured with a y multichannel spectrometer.Our experimental results allow us to verify that y activity fixed on particles with radii below 2.10-6 cm represents an average of 16 % of the total artificial radioactivity.