Risk factors

We will use tobacco smoking as an example of a risk factor that:

  1. Increases the incidence risk for a number of diseases; and

  2. Can be mitigated by interventions that reduce smoking prevalence.

Tobacco smoking

Similar to chronic diseases, we can define the prevalence of tobacco smoking in terms of initial prevalence, incidence (“uptake”) and remission (“cessation”).

The simplest risk factor will have two categories of exposure:

  • No exposure; and

  • Exposure.

However, since cessation of tobacco smoking does not immediately reverse all effects of exposure, we will increase the number of exposure categories so that the exposure can gradually return to baseline over a period of 20 years. In other words, we assume that it takes 20 years after quitting to recover the health risks associated with having never smoked. The exposure categories will therefore be:

  • No exposure (never smoked);

  • Exposure (currently smoking);

  • 0 years post-cessation;

  • 1 year post-cessation;

  • 2 years post-cessation;

  • …

  • 19 years post-cessation;

  • 20 years post-cessation; and

  • 21+ years post-cessation.

Upon cessation, an individual will progress through the post-cessation exposure levels and, 21 years later, their exposure category will be 21+ years post-cessation and they will have the same incidence risks as those individuals who have never smoked.

For each exposure level, we need the relative risk (or risk ratio) for each disease of interest. This is how the prevalence of the exposure will affect disease incidence, which in turn will affect the mortality and YLD rates in the MSLT.