Atmospheric new particle formation (NPF) has been observed to take place in practice all around the world. In continental locations, typically about 10-40% of the days are so-called NPF event days characterized by a clear particle formation and growth that continue for several hours, occurring mostly during daytime. The other days are either non-event days, or days for which it is difficult to decide whether NPF had occurred or not. Using measurement data from several locations (Hyytiala, Jarvselja, and near-city background and city center of Budapest), we were able to show that NPF tends to occur also on the days traditionally characterized as non-event days. One explanation is the instrument sensitivity towards low number concentrations in the sub-10 nm range, which usually limits our capability to detect such NPF events. We found that during such days, particle formation rates at 6 nm were about 2-20% of those observed during the traditional NPF event days. Growth rates of the newly formed particles were very similar between the traditional NPF event and non-event days. This previously overlooked phenomenon, termed as quiet NPF, contributes significantly to the production of secondary particles in the atmosphere.