The upper limit of the laser field strength in a perfect vacuum is usually considered as the Schwinger field, corresponding to ～1029W/cm2. We investigate such limitations under realistic nonideal vacuum conditions and find that intensity suppression appears starting from 1025W/cm2, showing an upper threshold at 1026W/cm2 level if the residual electron density in chamber surpasses 109cm-3. This is because the presence of residual electrons triggers the avalanche of quantum electrodynamics cascade that creates copious electron and positron pairs. The leptons are further trapped within the driving laser field due to radiation reaction, which significantly depletes the laser energy. The relationship between the attainable intensity and the vacuity is given according to particle-in-cell simulations and theoretical analysis. These results answer a critical problem on the achievable light intensity based on present vacuum conditions and provide a guideline for future hundreds of petawatt class laser development.