The effect of a high aerosol concentration on the collection efficiency of impactors has been studied. The flow was assumed to be dilute and two-way coupled. A particle source in cell model for the computational scheme was used. Results show that the effect of the particle interactions with the air on the collection efficiency is enlarged as the aerosol concentration is increased. It was also found that when the value of the Reynolds number or the overall particle sizes increased, the change of the collection efficiency by the change of the aerosol concentration also increased. The effect of the high aerosol concentration on the collection efficiency was found to be considerable even in slow flows when particles are large.