What is Molecular Filtration
Molecular filtration is a term used to describe filtration of chemical contaminants having sizes at the molecular scale. While particulate filters, such as HEPA filters, are effective in filtering particles down to the 0.3 micron size range (1 micron is 1/1,000,000 of a meter), they are ineffective against chemical contaminants such as gases and volatile organic compounds (VOCs) present in the vapor phase or as dissolved organics in the liquid phase. Molecular size is measured in Angstroms (1 angstrom is 1/10,000,000,000 of a meter). As an example, the size of a benzene molecule is approximately 6 Angstroms or 0.0006 microns.
Sources of Molecular Contamination
Sources of molecular contamination can exist both inside and outside building environments. Outdoor pollution sources include vehicle exhaust, combustion byproducts from manufacturing processes, emissions from process equipment and chemical supply lines, cross-contamination between manufacturing areas, chemical storage areas, waste management facilities, agricultural and animal farming. Indoor sources of pollution include human metabolic byproducts, off-gassing from building and construction materials, ozone and fugitive chemical emissions from electronic equipment and housecleaning solvents.
Types of Molecular Contamination
Molecular contamination may be classified in four categories each requiring different control strategies. These categories are as follows:
Toxic: A substance is said to be toxic if it exhibits the ability to cause damage to living tissue, impairment of the central nervous system, or in extreme cases, death when ingested, inhaled, or absorbed through the skin.
Corrosive: Those compounds which are likely to cause deterioration or damage to the interior of a building or its contents are considered corrosive. They may also have a detrimental effect on human occupants as well.
Irritant: Chemicals that can be said to cause discomfort, and potentially permanent damage, to an exposed person may be considered irritating. Many of the gases considered to be irritants produce symptoms of pain or discomfort to the eyes, skin, mucous membranes, or respiratory system.
Odorous: Materials that primarily affect the olfactory senses are considered odorous and usually carry negative connotations.
There are essentially three strategies used to control molecular contamination:
- Source control: Removal of the contaminant source or control of its emissions
- Dilution: Ventilation with clean dilution air
- Air cleaning: for either particles or gases, or both
Source control for outdoor air contaminants is often neither feasible nor practical; therefore, ventilation control should be the next option. However, this may not prove viable in all cases either as the use of large amounts of outdoor dilution air is neither cost-effective nor energy-efficient. Further, bringing in additional quantities of outside air could result in substituting one group of contaminants for another— those with sources outside the building for those internally generated. In areas with poor outdoor air quality, neither source or ventilation control can prevent the introduction of contaminants into a facility; therefore, air cleaning must be employed. Air cleaning is often used as an alternative to source control and ventilation. A gas-phase air filtration system can effectively reduce the concentration of the contaminant to levels that are at or below the level of detection for the monitoring techniques employed.