Effect of follow-up time on risk estimates: a longitudinal examination of the relative risks of leukemia and multiple myeloma in a rubber hydrochloride cohort

Am J Ind Med. 2002 Dec;42(6):481-9. doi: 10.1002/ajim.10139.


Background: Choice of follow-up time for an occupational cohort can influence risk estimates. We examined the effects of follow-up time on relative risk estimates for leukemia and multiple myeloma in a cohort of 1,845 rubber hydrochloride workers.

Materials and methods: We generated standardized mortality ratios (SMRs) for yearly follow-ups, beginning each study in 1940 and increasing study end dates from 1950 through 1996. We used Cox proportional hazards modeling to explore the effects of follow-up time on the exposure-response relationship.

Results: The SMR for leukemia rose to 13.55 in 1961 and fell nearly monotonically to 2.47 by 1996. Cox modeling suggested interaction between cumulative exposure and time since exposure. A longer time to peak risk was seen for multiple myeloma.

Conclusions: Because summary risk estimates change with follow-up time, exposure limits set using these estimates may not adequately protect workers. Consideration of appropriate follow-up time and use of more complex temporal models are critical to the risk assessment process.

MeSH terms

  • Benzene / adverse effects*
  • Cohort Studies
  • Dose-Response Relationship, Drug
  • Humans
  • Leukemia / chemically induced*
  • Leukemia / mortality
  • Longitudinal Studies
  • Male
  • Multiple Myeloma / chemically induced*
  • Multiple Myeloma / mortality
  • Occupational Diseases / chemically induced*
  • Occupational Diseases / mortality
  • Occupational Exposure*
  • Proportional Hazards Models
  • Risk Assessment
  • Rubber
  • Time Factors
  • United States / epidemiology


  • Rubber
  • Benzene