An endoscopic bite block (30, 31, 66), comprising: a central channel adapted for insertion into the mouth of a subject, the central channel comprising an upper wall having an upper outer surface comprising a top groove or a lowered platform region having an edge curved to shape the approximate curve of the subject's upper teeth and an upper inner surface; and a lower wall having a lower outer surface comprising a lower groove or a sloping stepped depression and a lower inner surface, the central channel further comprising an opening in the roof of the channel; a breath sampling channel built into the roof of the channel and configured to sample orally exhaled breath; a front plate (42, 44) merged with an outer end section of said central channel, said front plate (42,44) comprising an upper section and a lower section; said upper section comprising a groove in the top of the front plate (42, 44) of the bite block (30, 31, 66) with an oral breath directing scoop at the bottom of the groove, the groove terminating close to the sampling opening, a penetrable curtain disposed in the outer end section across the majority of the internal cross section of the central channel and substantially preventing flow of gases across the majority of the cross section of the central channel, the penetrable curtain having flaps formed by means of slits across the surface of the curtain, the penetrable curtain being attached to the front plate (42, 44) of the bite block (30,31, 66) around its periphery, and the penetrable curtain having a small hole, and a collection tube connectable to said breath sampling channel, said collection tube configured to receive the orally exhaled breath sampled by said breath sampling channel. a central channel adapted for insertion into the mouth of a subject, the central channel comprising an upper wall having an upper outer surface comprising a top groove or a lowered platform region having an edge curved to shape the approximate curve of the subject's upper teeth and an upper inner surface; and a lower wall having a lower outer surface comprising a lower groove or a sloping stepped depression and a lower inner surface, the central channel further comprising an opening in the roof of the channel; a breath sampling channel built into the roof of the channel and configured to sample orally exhaled breath; a front plate (42, 44) merged with an outer end section of said central channel, said front plate (42,44) comprising an upper section and a lower section; said upper section comprising a groove in the top of the front plate (42, 44) of the bite block (30, 31, 66) with an oral breath directing scoop at the bottom of the groove, the groove terminating close to the sampling opening, a penetrable curtain disposed in the outer end section across the majority of the internal cross section of the central channel and substantially preventing flow of gases across the majority of the cross section of the central channel, the penetrable curtain having flaps formed by means of slits across the surface of the curtain, the penetrable curtain being attached to the front plate (42, 44) of the bite block (30,31, 66) around its periphery, and the penetrable curtain having a small hole, and a collection tube connectable to said breath sampling channel, said collection tube configured to receive the orally exhaled breath sampled by said breath sampling channel.