Respiratory diseases infect millions of people every year, whether the disease be Influenza, the Rhinovirus, or SARS. While there is not a clear consensus on how much environmental factors affect these diseases, there is evidence to show that the environment can have major impacts of how infective these diseases are.
Studies conducted have shown that temperature and humidity have a significant impact on transmission. While some of these factors come from humans changing their habits as seasonal differences occur, some come from changes in air temperature and humidity; for example, cold dry winter air impairs the ability of the lungs to clear themselves. This was shown in a 1960s study in which mice were infected with influenza. When the humidity increased, transmission of influenza decreased. In a similar study with guinea pigs, when the relative humidity was at 20%, transmission of influenza was the greatest, while at 80% relative humidity, there was little to no transmission of the disease. While these tests both show that higher relative humidity lowers transmission of influenza, viruses such as adenovirus and rhinovirus are more stable and are able to transmit at these higher humidity levels. This appears to be due to differences between relative humidity, compared to temperature and actual humidity, which is the actual amount of water vapor in the air.
The effect of rainfall on the transmission of disease appears to be mixed. A study on RSV in Indonesia found that the more days it rained, the higher the infection rate. Similarly, a study in Malaysia concluded that the more days it rained, the higher the number of infections. Contrarily, a study in Taiwan found no association was between rainfall and disease transmission. Many other studies concluded by agreeing with either of the two sides, leaving no clear consensus.
While there are not many studies, a study on mice concluded that an increase in ventilation lowered transmission of influenza.
Box 1
Modes of person-to-person transmission of respiratory viruses
| Contact transmission | In both modes of contract transmission (direct and indirect), contaminated hands play an important role in carrying virus to mucous membranes. |
| Direct transmission | Virus is transferred by contact from an infected person to another person without a contaminated intermediate object (fomite). |
| Indirect transmission | Virus is transferred by contact with a contaminated intermediate object (fomite). |
| Droplet spray transmission | Virus transmits through the air by droplet sprays (such as those produced by coughing or sneezing); a key feature is deposition of droplets by impaction on exposed mucous membranes. |
| Aerosol transmission | Virus transmits through the air by aerosols in the inspirable size range or smaller; aerosol particles are small enough to be inhaled into the oronasopharynx and distally into the trachea and lung. |
Pica, Natalie, and Nicole M Bouvier. “Environmental Factors Affecting the Transmission of Respiratory Viruses.” Current Opinion in Virology, U.S. National Library of Medicine, Feb. 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3311988/#BX1.
Table 1
Modes of transmission of several human respiratory tract viruses.
| Virus | Family | Primary mode(s) of respiratory transmission |
| Adenoviruses | Adenoviridae | Contact, possibly droplet spray and/or aerosol (limited data) [2–4] |
| Influenza viruses | Orthomyxoviridae | Contact, droplet spray and/or aerosol (conflicting data) [5–9] |
| Human parainfluenza viruses (HPIV) | Paramyxoviridae | Uncertain (limited data) [10–12] |
| Metapneumovirus | Paramyxoviridae | Uncertain (limited data) [2] |
| Respiratory syncytial virus (RSV) | Paramyxoviridae | Direct and indirect contact [7,13], possibly droplet spray [14] |
| Rhinoviruses | Picornaviridae | Contact, droplet spray and/or aerosol (conflicting data) [7,15] |
| SARS coronavirus | Coronaviridae | Droplet spray and aerosol [2,4,16], possibly contact [17] |
Pica, Natalie, and Nicole M Bouvier. “Environmental Factors Affecting the Transmission of Respiratory Viruses.” Current Opinion in Virology, U.S. National Library of Medicine, Feb. 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3311988/#BX1
Citations(Jonah Kasdorf):
Connolly, M A. “Communicable Disease Control in Emergencies.” World Health Organization, 2005.
Pica, Natalie, and Nicole M Bouvier. “Environmental Factors Affecting the Transmission of Respiratory Viruses.” Current Opinion in Virology, U.S. National Library of Medicine, Feb. 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3311988/#BX1