A method to determine vapor phase corrosion load in paper and board manufacturing system is disclosed. The method allows optimizing oxidative biocide content in aqueous solutions, suspensions and slurries in the paper and board manufacturing system. The method is based on determination of redox values (rH) of the system during determined time period. A correlation between rH values exceeding a predetermined rH threshold values and corrosion load provides an estimate of increased or decreased corrosion load and allows timely changes to biocide dosage.