Exceptional filtration captures fine particles, including smog, and smoke, recommended for allergy and asthma sufferers. The Plus Carbon provides maximum odor and VOC reduction. MERV rated synthetic poly blend pleated media plus activated charcoal flat panel added to increase odor control Nominal Size: 20 x 25 x 4, Actual Size: 19 1/2 x 24 1/2 x 3 5/8 (19.5 x 24.5 x 3.63) Made in the USA: Proudly manufactured in the USA, ensuring quality and supporting local production Measure your air filter for the exact size and compare it to our actual size to ensure proper fit. Nordic Pure recommends changing 4" air filters every 3-6 months.