Activated charcoal filters are used in countless devices and processes to remove impurities from gases or liquids. Restorers frequently ask how long a charcoal filter is effective before it has to be replaced.
Unfortunately, there is no precise answer. To explain, we need to consider how activated charcoal functions.
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 (we will refer to them simply as the “contaminant”) get trapped in the pores.
As a liquid or gas (we will simply call this “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 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.
- Airflow. 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. Indeed, at very high temperatures (well over 212°F and under special conditions), contaminants are released from the carbon. This is how carbon is “activated.”
- 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. At some point 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 would be possible to estimate the life span of the carbon filter for the HEPA unit if the values of all these variables could be known accurately; however, it’s likely that any estimate will be so vague it will be of little value.
The best option? Rely on your sense of smell when using a HEPA unit to remove malodors. When the filter no longer traps odor, then it is time to replace the filter.
Mike Kerner is the senior scientist for Legend Brands. He has a master’s degree in chemistry from Purdue University. Kerner has worked in the industry for more than 28 years in capacities ranging from technical consulting to regulatory compliance to quality control standards. He understands every facet of chemistry from manufacturing efficiencies to formulation performance. He also has a talent for distilling complex scientific principles into information people can use.