Background and aim: For the assessment of lung function decline that leads to air-flow limitation, we want to know the reference ranges as well as the point representative values of change rates.
Method: For subjects who underwent more than 5 tests in the study period, %FEV₁ change rate(?%FEV₁) is calculated from the linear regression between age and %FEV₁.  From mean(?), standard deviation(?, SD) of  ?%FEV₁ of healthy non-smokers (HNS), the lower limit of normal (LLN) or the lower 5th percentile is calculated. Expected FEV₁ change rates (?FEV₁) and their LLN that corespond to sex, age and height are calculated from ?%FEV₁. The LMS method reference equations of  Japanese Respiratory Society 2013¹⁾ is used.
Results: 1955 male, and 5546 female HNS underwent for average 9.7 tests per subject between 2005 and 2019. The mean ?%FEV₁ of HNS is 0.00%/y, SD is 0.92%/y, and LLN is -1.53%/y.  ?FEV1 of average subjects are, for male (60 y.o, 169cm height),  mean -22.8ml/y, LLN -69.8ml/y, for female (60 y.o, 156cm ), mean-11.5 ml/y, LLN -45.2 ml/y.
Conclusion: The LLN as well as the mean of ?%FEV1, ?FEV1 are calculated from the lung function dataset of a large number of normal subjects, which can help us interprete the spirometries of aging population and those at the risk of obstructive lung diseases.

1) Kubota M, et.al. Respiratory Investigation. 2014;52(4 ):242-250