what is the yearly minimum amount of carboxyhemoglobin patient 29971 has until 5 months ago?
select min(labevents.valuenum) from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 29971 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'carboxyhemoglobin' ) and datetime(labevents.charttime) <= datetime(current_time,'-5 month') group by strftime('%y',labevents.charttime)