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How Long Does a Carbon Filter Last?
Activated charcoal filters are used in countless devices and processes to remove impurities from gases or liquids. An optional activated charcoal filter allows the Dri-Eaz DefendAir® HEPA 500 to be used in a wide of variety of situations where odor control is required. 

Customers frequently ask how long a charcoal filter is effective. Unfortunately there is no precise answer. Activated charcoal is a carbon-based substance with a large number of pores providing an extraordinarily large surface area. These pores adsorb, or trap, molecules. These molecules (the “contaminant”) get trapped in the pores. As a liquid or gas (“air”) passes over the charcoal, a certain percentage of a contaminant is retained on the surface of the carbon material. It might take several passes through a filter to remove enough of the contaminant so that it no longer causes odor problems.

The degree to which a specific contaminant becomes trapped on the charcoal depends on several variables:
  • Type of contaminant. Some molecules are easily trapped on the charcoal while others have a very poor attraction. For example toluene, a solvent with a very strong odor, is highly attracted to charcoal, but formaldehyde, a well-known indoor air pollutant, is poorly attracted.
  • Movement of the fluid. The contaminant is trapped only when the air containing it moves over the filter. As the airflow rate increases, more air is being cleaned but the contaminant also has less time to get trapped by the charcoal.
  • Temperature. In general the higher the temperature of the air, the less efficient the scrubbing action. The efficiency drops about 0.25% for every 1°C increase in temperature. (In fact, when exposed to very high temperatures, carbon will release the contaminants. This is how carbon is "activated" in the first place.) 
  • Humidity. Water molecules will compete with contaminant molecules for space in the pores. At some point there is so much water that the contaminant gets “squeezed out.”
  • Capacity. Eventually all the pores become filled with contaminant molecules. Think of a sponge holding all the water it can. This is the critical factor that determines how soon a carbon filter must be replaced.

It might be possible to estimate the life span of the carbon filter for the HEPA 500 if the values of all these variables could be known accurately; however, such an estimate would be difficult and of limited value.

The best option? Rely on your sense of smell when the HEPA 500 is being used to remove malodors. When the filter no longer traps odor, then it is time to replace the filter.
Mike Kerner, Legend Brands Chemical Product Specialist, has over three decades of experience in chemistry in capacities ranging from technical consulting to regulatory compliance to quality control standards.