Viruses

A virus is an extremely small infectious agent - much smaller than a fungus or bacterium - that must invade a living cell to reproduce (replicate). The virus attaches to a cell, enters it, and releases its genetic material inside the cell. The viral genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. Before it dies, however, the cell releases new viruses, which go on to infect other cells.

Modern air filtration devices such as face masks, HEPA filters, HME filters, etc., use synthetic or non-woven fabric’s as a filtration layer.  The filtration layer offers a barrier to particles - either mechanically or electro-statically - and thus particles entering the filter are removed and clean air is inhaled at the other side of the device.

Tests show that filter materials are very efficient at removing small particles (< 0.1 of um diameter) such as bacteriophage from air streams because electrostatic charge and Brownian motion trap these fine particles with in the matrix of the non-woven fabric. Larger particles (>1.0um in diameter) such as bacteria or fungi are also efficiently removed by mechanical filtration.  However filtration media are generally poor at removing particles in the range from 0.1um to 0.5um in diameter. Somewhat counter intuitive, these mid size particles are small enough to escape the forces of mechanical filtration yet are large enough to avoid being entrapped by electrostatic or Brownian motion.  These particles are often referred to as the Most Penetration Particle Size (MPPS).

Unfortunately, many enveloped viruses are in this range of dimension and thus can pass through filtration media.  In fact, Influenza virus (A, B, H1N1, H5N1) the SARS corona virus, pediatric Respiratory Syncitial Virus have a particle diameter between 0.08 micron and 0.3micron and thus are poorly removed by air filters.

Enveloped viruses

Although viruses have adapted to different host systems during evolution and many different viral strategies have developed, certain similarities can be found. For instance, many of the highly pathogenic viruses including influenza virus, HIV and others of worldwide epidemiological importance are said enveloped, meaning that they are surrounded by a lipid membrane. This viral membrane, which originates from the host cell during the exit process, is required to protect the viral genome and provide important functions for attachment, morphogenesis and transmission.

Mode of transmission

Viruses are transmitted in a variety of ways. Some are swallowed, some are inhaled, some are transmitted by the bites of insects and other parasites (for example, mosquitoes and ticks) and some are transmitted by sexual intercourse.
Probably the most common viral infections in humans are those of the nose, throat, and airways. These infections include sore throat, sinusitis, the common cold, and influenza. These diseases are often termed upper respiratory infections (URIs). In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngitis) or other airways deeper inside the lungs (bronchiolitis, bronchitis).