airflow obstruction happens first in distal airways

distal, smaller airways are impacted first when there's airflow obstruction
this manifests as reduced airflow at lower volumes (end of expiration)
this is why obstructive PFTs have characteristic appearance
end of expiration tapers much slower while the first part (FEV1) is less affected
$FEF_{25-75\%}$ are sometimes used to describe this decrease in small airway flow between 25% and 75% of exhaled FVC (lung volumes, capcities, normal flow volume loops definitions). However the measurements are widely variable and thus not used.¹

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