using IQR as a scale for a continuous independent variable I'm Trying to understand how associations between continuous exposures and health outcomes are typically reported.  When reading studies that report on the relationship between air pollution or temperature exposure and health outcomes I usually see the IQR being used as a scale of exposure. For example, a study might report "an IQR increase in X exposure is associated with an 5% increase in the risk of Y disease."  I'm not clear what this increase is relative to. Specifically, an IQR increase compared to what? no exposure? Or is it a comparison between those in the highest exposure category (75th percentile) to the lowest exposure category (25th percentile)? I'm not familiar with this work and would like to understand it so I can interpret it in simple terms -thanks in advance.
 A: Presuming that the relationship between the exposure and the health outcome is linear - an increase of one IQR of the exposure will always have the same effect on the outcome Y, no matter where you start from. This is because the regression line is linear - any point along the line (i.e. any value of the exposure) can be used as a reference. To make things concrete, you may consider the reference value the 25th percentile and then the regression coefficient represents the amount that Y is changed when the exposure is increased to the 75th percentile.
Relative risks (the effect measure you allude to in your question) make things a bit more complicated. Because risks (unlike odds) are constrained to be between 0 and 1, the size of the relative risk will depend on the baseline prevalence of the outcome. If the baseline prevalence of the outcome is high, the relative risk is biased towards 1 (the null value). Some advocate for the use of odds ratios rather than relative risks for this reason.
