Table 4. Hazards ratio (HR) of respiratory tract carcinomas in relation to combined levels of cigarette smoking status, and frequency of incense use, Singapore Chinese Health Study 1993-2005.

	Level of incense use
	Non-current users†	Low (night/intermittent)	High (day/all times)	P for trend
Upper respiratory tract Nasopharyngeal carcinoma
Never smokers
No. cases	30	57	14
HR (95% CI)*	1.0 (referent)	0.7 (0.5-1.2)	1.0 (0.5-2.0)	0.49
Ever smokers
No. cases	10	55	9
HR (95% CI)*	1.0 (referent)	1.4 (0.7-2.7)	1.4 (0.6-3.5)	0.38
Upper respiratory tract Non-nasopharyngeal carcinoma
Never smokers
No. cases	5	25	7
HR (95% CI)*	1.0 (referent)	2.2 (0.8-5.9)	3.3 (1.0-10.7)	0.04
Ever smokers
No. cases	14	82	17
HR (95% CI)*	1.0 (referent)	1.3 (0.7-2.4)	1.7 (0.8-3.5)	0.16
Lung cancer
Never smokers
No. cases	69	140	23
HR (95% CI)*	1.0 (referent)	0.8 (0.6-1.1)	0.7 (0.4-1.1)	0.09
Ever smokers
No. cases	90	429	70
HR (95% CI)*	1.0 (referent)	1.1 (0.8-1.3)	1.0 (0.7-1.4)	0.88

^*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).

^†Including both never and former users of incense.