Table 2.

The association between different smoking-related exposures and incident back pain*

Categories	n/N events	HR (95% CI)
Smoking status
Never	248 128/16 166	1 (ref)
Former	158 123/12 120	1.13 (1.10-1.16)
Current	32 259/3181	1.50 (1.45-1.56)
P-value for trend	P < 0.000	1.19 (1.17-1.21)
Number of cigarettes smoked daily
None	303 681/19 936	1 (ref)
Up to 10	38 702/2916	1.12 (1.08-1.17)
Between 11 and 20	70 267/6042	1.24 (1.20-1.28)
Between 21 and 30	17 187/1650	1.37 (1.30-1.44)
More than 30	8673/923	1.45 (1.36-1.55)
P-value for trend	P < 0.000	1.11 (1.10-1.12)
HR per 5 units	P < 0.000	1.05 (1.05-1.06)
HR per 10 units	P < 0.000	1.10 (1.11-1.12)
Pack-years of own smoking
None	306 012/20 156	1 (ref)
Up to 10	36 398/2411	1.03 (0.98-1.07)
Between 11 and 20	35 975/2867	1.17 (1.13-1.22)
Between 21 and 30	25 697/2387	1.32 (1.27-1.38)
More than 30	34 428/3646	1.45 (1.40-1.50)
P-value for trend	P < 0.000	1.10 (1.09-1.10)
HR per 5 units	P < 0.000	1.04 (1.04-1.04)
HR per 10 units	P < 0.000	1.08 (1.07-1.09)

*All hazard ratios (HRs) and their 95% confidence intervals (CIs) were derived from a Cox proportional hazards regression model adjusted for age, sex, ethnicity, body mass index, educational level, Townsend Deprivation Index, and sedentary behavior.