Excess mortality and morbidity from respiratory illnesses is commonly reported during episodes of unusually low and high temperatures. Reasons behind increased risk of adverse respiratory events during environmental cold and heat exposure are however poorly understood. The aims of this presentation are to i) highlight the main effects of cold and hot air exposure on airway function and pulmonary ventilation, and ii) discuss potential functional and clinical consequences of such effects (particularly in vulnerable groups, such as the very young, the very old and those living with a chronic lung condition).

One of the primary roles of the lungs is to condition inhaled air to 37°C and 100% humidity before it reaches the alveoli. When cold air is inhaled, excess heat and water loss within the airways may modify properties of the airway surface lining fluid, rendering it hyper-osmolar. In those individuals with inflamed airways (such as patients with asthma), hyper-osmolarity of the airway surface lining fluid triggers an inflammatory cascade that ultimately leads to narrowing of the airways (bronchoconstriction). This effect is particularly apparent at times of high ventilatory demand (i.e., during physical activity and exercise). In healthy individuals, inflammatory-mediated bronchoconstriction is usually not present. However, reflex bronchoconstriction (via cooling of the facial skin and/or activation of muscarinic receptors within the airways) may occur during exposure to sub-freezing temperatures in both, healthy and clinical populations.

In the heat, conditioning of inhaled air is easily achieved, limiting the risk for osmotically-driven bronchoconstriction. A rise in temperature above normal body temperature (as might occur during heatwaves or prolonged exercise) can however cause reflex (C-fibre mediated) bronchoconstriction, particularly in those individuals suffering from allergic asthma. Further, during uncompensable heat stress, elevations of body core temperature of 1 to 2°C lead to hyperventilation (both at rest and during exercise). While hyperthermic-hyperventilation has been widely studied in young and athletic populations, far less is known about its occurrence and functional/clinical impact in older and patient populations.

As earth is warming and cold and hot waves become more frequent, severe and long lasting, more mechanistic work is required to help develop effective strategies to protect the most vulnerable against adverse respiratory events.