In that study, an increase in ambient temperature of 10oC resulted in a shift in mass median aerodynamic diameter (MMAD) of around 0.2µm, from below 3.3µm to almost 3.5µm. Changes of similar magnitude also resulted when the RH was increased by 20 percentage points or when impactor temperature was reduced by 5oC.
Why do we see such significant effects? For nebulizers, we can point to mechanisms such as the impact of ambient temperature on the properties of the formulation, most specifically viscosity and, by extension, the droplet formation process. Ambient temperature and RH both influence air density, an important variable with respect to the aerodynamic performance of the impactor, while impactor temperature defines the thermal mass of the instrument and its ability to promote droplet evaporation.
With DPIs, electrostatics can be particularly problematic due to its influence on device emptying, de-aggregation of the dose, and deposition within the impactor. At higher RH, water in the atmosphere serves to dissipate electrostatic charge, eliminating accumulation; however, hygroscopicity can become an issue as water levels in the atmosphere rise. RH control in the region of 40 – 60% is typically the target, to balance these competing effects.
Pressurised MDIs share some of the issues associated with both DPIs and nebulizers in that they are subject to significant triboelectrification, but charge dissipation is highly variable depending on the formulation properties and the material of construction of the device. Droplet growth can occur at higher RH levels, though this may be a transitory effect and, as with nebulizers, droplet evaporation can also occur in the cascade impactor.
In summary, the importance of environmental conditions is more easily recognised than fully understood and may vary significantly between products. Of course, it can also be potentially problematic to exert sufficient control over the test environment due to financial and practical constraints.
What solutions and strategies do you think are particularly helpful?
Either climate control of the laboratory or an environmentally controlled chamber of some type would be necessary to deal with ambient temperature and RH variability.
The feasibility of full lab control is often geographically dependent because both capital and running costs may be prohibitive where there are significant seasonal and diurnal temperature fluctuations and swings in RH. Even in areas with less variation, it can be difficult to achieve uniformity across a large space due to factors such as heat release by in-use equipment, drafts, windows, and the location of air-conditioning vents.
