Environmental temperature and relative humidity influence exhaled breath condensate pH

To the Editors:

Exhaled breath condensate (EBC) is used to obtain information about the composition of airway surface lining fluid 1. Acidification of EBC has been reported in different inflammatory airway diseases 2. Interpretation of data is compromised by the fact that condensate pH shows day-to-day variability, even in healthy subjects 3. Coating of condensing surfaces 4 and salivary contamination of the condensate 5 have been proposed to influence exhaled biomarker levels. It was our aim to investigate whether environmental temperature and relative humidity could influence condensate pH and whether they could be responsible for the observed variability.

EBC was collected from 12 healthy subjects with an R-Tube condenser (Respiratory Research Inc., Charlottesville, VA, USA) before and after the forecasted arrival of three cold, humid and two warm, dry meteorological fronts. In summer, on three occasions the room temperature and relative humidity paralleled the outside conditions; in winter, on two occasions room temperature and relative humidity were maintained at 26°C and between 47–52%.

EBC pH was determined using the carbon dioxide (CO₂) standardisation method, as described previously 3. Briefly, pH and CO₂ were measured using a blood gas analyser (ABL 520; Radiometer, Copenhagen, Denmark) five times consecutively after 1-s CO₂ load to the sample between each measurement. A pH–CO₂ plot was created from the results. The pH at 5.33 kPa CO₂ partial pressure was calculated using the logarithmic regression equity obtained from the plot.

The arrival of forecasted meteorological fronts caused a pronounced change both in temperature and relative humidity of ambient air (table 1⇓). In summer, when the study room conditions paralleled the weather conditions outside, EBC pH decreased significantly with the humid, cold fronts and inversely increased with the dry, warm front. In winter, when room conditions were controlled, EBC pH was not affected significantly by either the humid, cold or the dry, warm fronts. The coefficient of variation of EBC pH was 3.0±1.3 under uncontrolled room conditions in summer and 1.8±0.9 under controlled temperature and relative humidity of the room in winter (p<0.02). There was no correlation between pH and volume of condensates.

The results have two possible readings, one clinical and the other pathophysiological. From a clinical point of view the results suggest that controlling room temperature and relative humidity should be part of standardisation of EBC collection. From a pathophysiological point of view the results are interesting as they may contribute to the explanation of catching a common cold. It is not clear why the microbes that are present continuously in the air and on the mucous membranes cause respiratory tract infections that are more likely in cold weather. Compared with the stability of blood pH, changes ≥0.5 may occur in the pH of EBC and probably, therefore, the airway surface lining fluid. A relative important change in pH may result in a decrease in the ciliary beat 6 or in the activity of immune cells. The deterioration in the function of the nonspecific immune barrier may allow the viruses to invade the tissues.

It is widely accepted that meteorological factors can trigger several respiratory diseases. Asthma attacks have been related to: the rupture of pollens during thunderstorms 7; higher concentrations of sulphur dioxide, nitric oxide and carbon monoxide in the air; and lower temperature and sunshine hours 8. Exacerbations of chronic obstructive pulmonary disease have been demonstrated to be more frequent in cold weather 9.

In summary, the results suggest that environmental temperature and relative humidity contribute to the variability of condensate pH.

Statement of interest

None declared.