PNIF has been recommended and been used as an outcome parameter in allergen immunotherapy trials [
19,
20], pharmacological trials [
21,
22], nasal allergen challenges [
23,
24], before surgical interventions [
25,
26], and is also a feasible tool in assessing nasal patency in both children [
27] and adults [
28,
29]. This was the first study to evaluate peak nasal inspiratory flow as an outcome parameter in an allergen exposure chamber. Overall, 86 subjects participated in 203 individual challenges with either grass pollen, birch pollen, house dust mite material or placebo. At baseline, male subjects reached a PNIF of 174.2 ± 59.9 L/min and female subjects of 126.3 ± 31.0 L/min. Measurements were taken in a seated position, as there exists no significant difference to standing position [
30], and the best of at least two successful measurements was noted due to no additional benefit in a third trial [
15]. Reproducibility and no demand for priming exposures were previously reported [
14]. Our results confirm a study by Denguezli Bouzgarou et al. who found almost exact same values in a healthy North African population with a mean PNIF in male subjects of 174 ± 54 and 126 ± 33 L/min in female subjects [
31]. Looking at data for a European population our values were lower than data obtained by Åkerlund et al. [
32], but comparable to findings from Ottaviano et al. with a PNIF of 143 ± 48.6 L/min for male and 121.9 ± 36 L/min for female [
33]. A study by Klossek et al. in a French population found clearly lower normal ranges in PNIF though. Even when only reporting the values obtained from the subjects, who reported no nasal discomfort at all, men had a mean PNIF of 100.3 ± 43.6 L/min and women of 79.3 ± 32.2 L/min [
34]. However, an explanation for these low values was not found. The greatest reduction in PNIF was elicited by HDM in our study, followed by grass pollen and birch pollen. PNIF also decreased mildly in the placebo group, even when no patient in the chamber was exposed to an allergen. Whether the decline results apart from the placebo effect itself, from decreasing patients’ effort during the exposure, increased osmolarity of nasal mucus due to increased ventilation from the measurements, or despite 55% humidity too dry air, needs to be further investigated. Standard deviation of some results for absolute PNIF values exceeded the mean value caused by the unequal distribution. Hence, it is of utmost importance to compare the relative reductions. Decreased PNIF is known in HDM allergy as allergic subjects usually present with nasal obstruction [
35]. However, little is known about the differences in nasal symptoms elicited by different airborne pollen. In our challenges PNIF decreased in subjects exposed to grass pollen much greater in both absolute and relative values than in subjects exposed to birch pollen. Nonetheless, both kinds of pollen had in common that the more the pollen concentration increased the more PNIF reduction was induced. These results imitate the conditions in nature as described by Caillaud et al. who described a linear relationship between birch pollen concentration and symptoms elicited until symptom severity reaches a plateau when a certain threshold concentration is exceeded [
36]. As demanded by a recently published position paper from the European Academy of Allergy and Clinical Immunology (EAACI) it is important to compare the obtained results between the existing exposure chambers [
37]. To the authors knowledge only two studies conducted in an Environmental Exposure Unit (EEU) in Kingston, Ontario have used PNIF as an outcome parameter in clinical trials [
38,
39]. Both studies were clinical evaluations of the EEU for birch pollen and grass pollen exposure, respectively. Focusing just on the reported PNIF data for provocations with grass pollen, the mean reduction of PNIF after 180 min of exposure compared to baseline to either 2500 or 3500 grains/m
3 grass pollen (
Lolium perenne) was 29.8 and 42.9 L/min, respectively, resulting in a relative reduction of 30.4 and 34.2%, respectively. These results match our findings with a PNIF reduction of 35.2 L/min (relative reduction 29.3%) after 120 min exposure to 4000 grains/m
3 of grass pollen compared to baseline. However, allergic patients were not provoked to placebo in the EEU, thus the effect of the chamber itself to allergic subjects is unknown. Furthermore, the technology of pollen distribution is totally different in both chambers. Whereas in the EEU and most of the other existing chambers pollen gets distributed via fans all over the exposition room, the GA
2LEN chamber provides an individual exposure to every subject giving an exact knowledge of the concentration every test subject got exposed to. Hence, even when using the same allergen concentration the results might not be directly comparable. Both chambers provoked less reduction in PNIF during challenges with birch pollen. That is why it can be suspected that birch allergy elicit less nasal congestion and other symptoms are more present. This needs to be further evaluated. In our study, we found moderate positive correlations between PNIF and weight, height and oral peak inspiratory flow. Even though some publications denied a correlation between PNIF and weight [
32] or PNIF and height [
40], other studies confirmed these associations, especially for PNIF and PEF [
41‐
43]. In our study PNIF and subjective nasal symptoms were found to correlate inversely with a Spearman’s rank correlation coefficient r
s = −0.59 between PNIF and TNSS, and r
s = −0.42 between PNIF and nasal congestion score. Other studies, that were using exactly the same TNSS as we did, computed correlations from −0.50 to −0.62 between PNIF and TNSS, confirming our analysis and thus consolidate the usefulness of PNIF as an objective control parameter for subjective symptoms [
44,
45]. Furthermore, the publication from Ellis et al. reported a weak to moderate negative correlation from −0.32 to −0.37 between PNIF and subjective scoring of nasal congestion, which can be validated and even enhanced with data obtained in the GA
2LEN chamber [
38]. The correlation between PNIF and VAS of overall subjective symptoms was found to be at −0.36 in the GA
2LEN chamber, thus being in the range of already published correlations of −0.39 to −0.48 between PNIF and VAS [
17,
28,
29]. However, these studies focused only on the VAS of nasal obstruction in particular. Hence, our findings provide additional information about the relation of PNIF and the actual patient’s perception of their overall symptom severity, which possibly represents real-life conditions more accurately.