The incorporation of intrinsic point defects into a growing crystal is affected by the presence of impurities that can react with vacancies and self-interstitials. The critical value of the ratio of the growth rate, V, to the axial temperature gradient, G, (V/G ratio) that separates the interstitial growth mode from the vacancy growth mode, is shifted by impurities, and this effect can be described by simple analytical expressions. Some impurities, such as oxygen, nitrogen, and hydrogen, trap vacancies and cause a downward shift in the critical V/G ratio (and also a fast increase in the fraction of trapped vacancies, on lowering T). Other impurities, like carbon, trap self-interstitials, and cause an upward shift in the critical V/G ratio (and also an increase in the fraction of impurity interstitials, on lowering T). The impurities affect both the incorporation and agglomeration stages of micro-defect production.