Incidence of respiratory infections such as common cold exhibits seasonal fluctuations. In most places, there is a peak in respiratory illness during the winter months with a wide range of URTIs (upper respiratory infections) caused by more than 200 different viruses.
Acute upper respiratory viral infections have traditionally been associated with winters and this may be the origin of the term ‘common cold’, which implies exposure to cold.
A few factors are specific to cold weather e.g. overcrowding, closed environments, lack of personal hygiene, humidity with lowered temperatures. These factors are relevant in cases of schools and residential institutions and are important in transmitting and spreading the viruses.
However, overcrowding only cannot be held responsible for all the respiratory infections. There are other factors involved including human body response to the changes in temperature and humidity in the air and longer survival of pathogens in cold and humid air. Also, your body is used to functioning in a certain temperature and once the season changes, your body takes time to re-adapt which makes your immune system vulnerable to infections and viruses.
So while cold weather means remaining confined to indoors, hot weather means more outdoor activities, which can cause allergies to flare. Pollen is another factor that aggravates respiratory problems, irritating existing conditions such as asthma or causing new problems. Headaches and heatstroke become common coupled with viral infection due to dehydration. These cases send people to the doctor just as much as cases of the flu or colds do in winter.
Our nose and mouth is exposed to the external air during the process of breathing. Every day each one of us warms, filters and humidifies approximately 14, 000 litres of air. This air contains suspended particulate matter contaminated with viruses, bacteria and fungi. Thus, the upper airways are always the most commonly infected area of the body.
The nose and upper airways have two major defenses against infection: the physical barrier of a continuously moving layer of mucus that lines the airway; and secondly, a local immune response involving white blood cells such as natural killer cells. Airway cooling compromises both these mechanisms of respiratory defense. There is a decrease in the mucociliary clearance and white cell activity at lower temperatures.
Since respiratory infections are spread primarily in air-borne droplets of respiratory mucus by coughs and sneezes, and by objects touched by hands contaminated with respiratory mucus, proximity with the patient and lowered defence mechanism results in frank URTI. A rapid decrease in air temperature associated with a spell of cold weather may also cause conversion of subclinical infections to clinical infections. In addition, the cold air exposure is also likely to increase the susceptibility of the uninfected population to infection by changes in nasal respiratory defence as described above.
Thus, the germs survive for a longer time in winters, coughing, sneezing and overcrowding makes it easy for them to become airborne and the lowered defence of nasal passages form a good medium for infection.
Hence, in order to minimize the spread of ‘colds’ during change of season, it is most important to cover that cough or sneeze with a tissue or handkerchief and not hands, washing hands, avoiding crowded places and avoiding shaking hands with a person with a cold.
Dr Bela Sharma,
Author is senior consultant, internal medicine, Fortis Memorial Research Institute