吁 has two distinct readings: yù (
籲) means to appeal or call out — used in formal contexts like
呼吁 (hūyù, to appeal to). As xū, it means a sigh or hush, as in
长吁短叹 (cháng xū duǎn tàn) — to heave sighs. The traditional form
籲 is used for the yù reading.