Relative risk (RR) with 95% confidence interval (95% CI) of developing seropositive rheumatoid arthritis for 18–70 years old female and male ever-smokers compared with never-smokers by duration of smoking
| Number of years of smoking | Number of exposed cases | RR* | 95% CI |
|---|---|---|---|
| *Relative risk adjusted for age, residential area, and sex. | |||
| <10 | 26 | 0.8 | 0.4 to 1.4 |
| 10–19 | 36 | 0.8 | 0.5 to 1.3 |
| ⩾20 | 217 | 2.6 | 1.8 to 3.6 |