Table 5. Hazards ratio (HR) of respiratory tract carcinoma by histological subtype in relation to frequency of incense use, Singapore Chinese Health Study 1993-2005.

	Level of incense use
Histology and site	Non-current users †	Low (night/intermittent)	High (day/all times)	P for trend
Squamous cell carcinoma
All respiratory tract‡
No. cases	47	226	59
HR (95% CI)*	1.0 (referent)	1.2 (0.9-1.6)	1.8 (1.2-2.6)	0.004
Upper respiratory tract
No. cases	18	97	22
HR (95% CI)*	1.0 (referent)	1.4 (0.8-2.3)	1.9 (1.0-3.6)	0.049
Lower respiratory tract
No. cases	29	129	37
HR (95% CI)*	1.0 (referent)	1.1 (0.7-1.6)	1.7 (1.0-2.8)	0.03
Non-squamous cell carcinoma§
All respiratory tract‡
No. cases	171	562	81
HR (95% CI)*	1.0 (referent)	1.0 (0.8-1.1)	0.8 (0.6-1.1)	0.16
Upper respiratory tract
No. cases	41	122	25
HR (95% CI)*	1.0 (referent)	1.0 (0.7-1.4)	1.2 (0.8-2.1)	0.50
Lower respiratory tract
No. cases	130	440	56
HR (95% CI)*	1.0 (referent)	0.9 (0.8-1.2)	0.7 (0.5-1.0)	0.047

^*Adjusted for age at recruitment, year of interview, gender, dialect group, level of education, body mass index, number of cigarettes/day, number of years of smoking, time since smoking cessation, alcohol consumption, dietary intakes of isothiocyanate, beta-cryptoxanthin, lutein, lycopene and summed Chinese preserved foods, and for women, number of children (see details in the Materials and Methods)

^†No current use comprise never and ex-users of incense.

^‡The sum of all respiratory tract carcinomas may differ from the total number of cases listed in Tables 2 and 3 since the present table counted the first incident cancer for a given individual.

^§Non-squamous cell carcinoma included adenocarcinoma, small cell carcinoma, undifferentiated, other and unspecified carcinoma.