Relative risk (RR) with 95% confidence interval (95% CI) of developing seropositive rheumatoid arthritis for 18–70 years old female and male subjects who have smoked cigarettes for ⩾20 years compared with never-smokers by intensity of smoking
| Number of cigarettes smoked a day | Number of exposed cases | RR* | 95% CI |
|---|---|---|---|
| *Relative risk adjusted for age, residential area, and sex. | |||
| 1–5 | 15 | 1.6 | 0.7 to 3.7 |
| 6–9 | 24 | 2.5 | 1.3 to 4.7 |
| 10–19 | 103 | 3.0 | 2.0 to 4.6 |
| ⩾20 | 75 | 2.4 | 1.5 to 3.7 |