Dear editor
I have read the letter by Sun et al and I am grateful to the honourable authors of this letter.
Several factors affect heart rate variability. Some of these factors are known and some are unknown and the effect of each of them is different on heart rate. In our study, we have considered important and influential factors such as age, sex and disease or drugs that affect heart rate and its variability. However, there may be other known or unknown factors that are beyond our control such as air pollution and its suspended particles. Considering that all participants in our study were resided in the southwestern part of Tehran (capital city of Iran), their exposure to air pollution is likely to be similar, and this may not change the results of the study. The impact of this factor should be investigated in a research which is designed to this purpose.
Disclosure
The author reports no conflicts of interest in this communication.