THE RISK OF CHRONIC EXPOSURE

A.  BACKGROUND

       1.  CHRONIC EXPOSURE

            Chronic health risks are typically caused by the long-term
            exposure to a harmful substance (or substances).   Health 
            effects are divided into two general categories:  cancerous
            and non-cancerous (or toxic).  

       2.  CHRONIC DEFICIENCY

            In some cases, the long-term deficiency  of an element in the
            diet can also have harmful effects on human health.
 

B.  EXPOSURE PATHWAYS

       1.  INGESTION

            The intake of substances contained in food or drink  
 

  
       2.  INHALATION

            The intake of substances contained in the air we breathe

 

       3.  DERMAL CONTACT 

            Absorption of substances that come in contact with skin 

 

C.  RISK CHARACTERIZATION

       1.  NON-CARCINOGENIC

            Non-carcinogenic compounds are assumed to have a lower
            threshold  level below which no adverse health effects occur.
              

  
       2.  CARCINOGENIC

            Carcinogenic compounds are assumed to present a risk at
            any level.  The risk is expressed in terms of the probability
            that a given exposure will cause an increase in the cancer
            rate (usually an increased risk of 1:1,000,000 is considered
            acceptable).

D.  EXPOSURE ASSESSMENT

       1.  DOSE-RESPONSE CURVES

            a.  Graphs showing the response of organisms to various
                 levels of  exposure to a given substance 

                     Carcinogenic vs. non-carcinogenic dose-response curves

                  A "full" dose-response curve for substances that have a
                  beneficial effect on health at low concentrations
 

            b.  Generally based on animals experiments carried out at
                 high doses for short durations of time.

           
       2.  DATA EXTRAPOLATION

            There are three steps of data extrapolation to derive a
            standard for a given substance under a given exposure
            pathway.  Each of these steps involves a high degree of
            uncertainty, so conservative assumptions are made.        

            a.  An organism response must be extrapolated from very
                 high to low doses (usually 1,000 to 10,000 times lower) 

            b.  An organism response must also be extrapolated from
                 short to long time periods (the goal is to determine the
                 chronic risk from 70 years of exposure at low doses)

            c.  Results must be extrapolated from animals to humans:
                 the highest level of exposure that produces no effect in
                 test animals is multiplied by 100 as an arbitrary margin
                 of safety 

       3.  WHAT STANDARDS MEAN

            Example:  if you drink water that contains a carcinogenic
            chemical at its MCL (Maximum Contaminant Level) for 70
            years, you supposedly  increase your risk of getting cancer
            by one in a million.   This is a small increase in risk given
            that the background cancer rate in the U.S. is one in four!
            (we are exposed to many carcinogens daily).

       4.  COMPLEXITIES 

            a.  Standards represent the risk for the "average" person,
                 but individuals react differently to same exposure level

            b.  Some substances are beneficial to human health at low
                 doses but toxic at higher doses (e.g., fluoride)

            c.  There are very few studies that show the combined risk
                 caused by two or more substances

 Return to Geology 100 Home Page