A system that reduces a level of substances in air within an environment, wherein the environment comprises a closed or partially open environment and wherein the substances are selected from the group consisting of pathogens, allergens and odor-causing agents, the system comprising: an inlet passage configured to receive inlet air from the environment, the inlet air comprising an initial level of the substances; a humidifier configured to generate water vapor to be added to the inlet air; a humidifier input configured to receive water vapor from the humidifier; a mixer portion where the inlet air from the inlet passage and water vapor from the humidifier are mixed to form a homogenous air/vapor mixture; a treatment chamber wherein the air/vapor mixture is subject to a photocatalytic oxidation treatment, the treatment chamber comprising: a plurality of active cell panels, the active cell panels having at least a portion of their surfaces coated with photocatalytic material; one or more ultraviolet light sources interspersed between the plurality of active cell panels such that UV light emitted from the one or more UV light sources impinges on photocatalytic material; wherein the combination of the one or more active cells, the one or more ultraviolet light sources, and the air/vapor mixture is adapted to generate a concentration of ozone between about 0.0001 and 0.0070 ppm, a concentration of hydrogen peroxide between about 0.25 and 0.45 ppm, and hydroxyl radicals and super ions that oxidize or decompose the substances such that the substance level in the treatment chamber is reduced; a controller; and a humidity sensor positioned between the mixer portion and an outlet passage of the treatment chamber, the humidity sensor configured to sense the humidity level of the air/vapor mixture and provide a feedback humidity signal to the controller, the controller being configured to provide a humidifier control signal to the humidifier to adjust water vapor output of the humidifier, based on the received feedback humidity signal, to bring the humidity of the air/vapor mixture to between 75 to 100 percent without humidity saturation or condensation, wherein the outlet passage is configured to allow treated air/vapor to exit the treatment chamber and be distributed into the environment, and wherein a residual amount of the ozone, hydrogen peroxide, hydroxyl radicals, and super ions continue to oxidize or decompose substances in the air or on surfaces within the environment until the residual amounts of ozone, hydrogen peroxide, hydroxyl radicals and super ions disassociate. an inlet passage configured to receive inlet air from the environment, the inlet air comprising an initial level of the substances; a humidifier configured to generate water vapor to be added to the inlet air; a humidifier input configured to receive water vapor from the humidifier; a mixer portion where the inlet air from the inlet passage and water vapor from the humidifier are mixed to form a homogenous air/vapor mixture; a treatment chamber wherein the air/vapor mixture is subject to a photocatalytic oxidation treatment, the treatment chamber comprising: a plurality of active cell panels, the active cell panels having at least a portion of their surfaces coated with photocatalytic material; one or more ultraviolet light sources interspersed between the plurality of active cell panels such that UV light emitted from the one or more UV light sources impinges on photocatalytic material; wherein the combination of the one or more active cells, the one or more ultraviolet light sources, and the air/vapor mixture is adapted to generate a concentration of ozone between about 0.0001 and 0.0070 ppm, a concentration of hydrogen peroxide between about 0.25 and 0.45 ppm, and hydroxyl radicals and super ions that oxidize or decompose the substances such that the substance level in the treatment chamber is reduced; a plurality of active cell panels, the active cell panels having at least a portion of their surfaces coated with photocatalytic material; one or more ultraviolet light sources interspersed between the plurality of active cell panels such that UV light emitted from the one or more UV light sources impinges on photocatalytic material; wherein the combination of the one or more active cells, the one or more ultraviolet light sources, and the air/vapor mixture is adapted to generate a concentration of ozone between about 0.0001 and 0.0070 ppm, a concentration of hydrogen peroxide between about 0.25 and 0.45 ppm, and hydroxyl radicals and super ions that oxidize or decompose the substances such that the substance level in the treatment chamber is reduced; a controller; and a humidity sensor positioned between the mixer portion and an outlet passage of the treatment chamber, the humidity sensor configured to sense the humidity level of the air/vapor mixture and provide a feedback humidity signal to the controller, the controller being configured to provide a humidifier control signal to the humidifier to adjust water vapor output of the humidifier, based on the received feedback humidity signal, to bring the humidity of the air/vapor mixture to between 75 to 100 percent without humidity saturation or condensation, wherein the outlet passage is configured to allow treated air/vapor to exit the treatment chamber and be distributed into the environment, and wherein a residual amount of the ozone, hydrogen peroxide, hydroxyl radicals, and super ions continue to oxidize or decompose substances in the air or on surfaces within the environment until the residual amounts of ozone, hydrogen peroxide, hydroxyl radicals and super ions disassociate.