<p>The article presents a unique experience of establishing local maximum permissible concentrations (MPC_loc) for pollutants in the soil. Arsenic MPC_loc were found in two soil types contrasting in stability (Mollic Leptosols Eutric and Haplic Cambisols Eutric) in the territory (Karachay-Cherkess Republic, Urup District) of a positive geochemical anomaly of arsenic (As) content in soils. In the laboratory model experiment, the studied soils were contaminated with different doses of As (As₂O₂): 1.5, 2, 5, 10, 25 of background concentrations. The most sensitive and informative biological indicators of the soil (phytotoxicity, enzymatic activity and the total number of bacteria) were determined. The study defines for the first time the doses of As ecotoxicity in Mollic Leptosols Eutric and Haplic Cambisols Eutric. Results revealed a reduction of 5%, 10%, 25%, and 50% of key bioindicators of soil condition. The integral indicator of the biological state of the soil was calculated and the concentrations of As at which the ecological functions of the soil are disrupted were determined according to the most sensitive and informative indicators mentioned above. MPC_loc As in Mollic Leptosols Eutric is 21.2&#xa0;mg/kg (with a background content of 10.5&#xa0;mg/kg), in Haplic Cambisols Eutric — 19.7&#xa0;mg/kg (with a background content of 9.8&#xa0;mg/kg). Unlike the current unified standards, the developed MPC_loc consider the regional geochemical background and the sensitivity of the soil biota, which allows for an objective distinction between natural anomaly zones and anthropogenic pollution.</p>