1. abnormal chest X ray, PNS view 2. previous surgical or endoscopic antireﬂux procedure 3. previous aerodigestive malignancy or Barrett's oesophagus 4. current smokers or ex-smok-ers (deﬁned as those who quit smoking <3 months prior to study enrolment or those who have quit, but have a 20 pack year smoking history) 5. Malignancy History (lung cancer, esophageal cancer etc.) 6. upper respiratory infection within 8 weeks of study enrolment 7. currently using a PPI, H2 blocker, beta-blocker, angiotensin-converting enzyme inhibitor, corticosteroid, methylxanthine, inhaled beta-agonist, anti-inﬂammatory agent or anticholinesterase drug at time of enrolment