In certain exemplary embodiments, an air purifier comprises a housing defining an enclosure and having an air entrance and an air exit; a particulate filter; a NCCO filter material configured to adsorb and decompose at least one gaseous pollutant; an AOG configured to generate at least one oxidant; an oxidant remover configured to remove at least one oxidant; a fan unit configured to generate airflow from the air entrance to the air exit; wherein the particulate filter, the NCCO filter material, the AOG, the oxidant remover and the fan unit are positioned within the enclosure such that during operation, a flow of air passes from the air entrance to the air exit through the particulate filter and the NCCO filter material along a direction of the flow of air. In certain embodiments, the air purifier may ensure safety to users while efficiency in removing contaminants can be greatly improved.