An activated carbon filter that treats a gas stream having a varying relative humidity, the gas stream contaminated with at least one of formaldehyde, an acid gas and an alkaline gas. The filter includes a plurality of activated carbon granules mixed together in a carbon bed filter. A first portion of the granules supports both manganese dioxide and potassium hydroxide. A second portion of the granules supports both potassium iodide and potassium hydroxide. A third portion of the granules supports phosphoric acid. The different portions operate to catalyze the oxidation of formaldehyde and other contaminants at varying humidity. The potassium hydroxide optionally chemisorbs any carboxylic acid by-products of the formaldehyde. Also provided is a method to treat a gas stream with the filter.