At the end of May I posted an article that reviewed the routes of exposure by which people are exposed to PCBs.  After some general discussion, the article focused on the total PCB intake for three elementary school-aged child receptors who attended schools with different indoor air PCB concentrations:

• A school with a relatively low 100 ng/m³ (nanograms per cubic meter) of PCBs in air;
•  A second school with 300ng/m³ of PCBs in air (this is the EPA Public Health Level); and
•  The third school with 400 ng/m³ of PCBs in air, 33% greater than the EPA Public Health Level.

The article calculated a total average daily dose of PCBs for these different student receptors by adding the amount of PCBs they ingested each day from food (this worked out to be 11.8 ug/day, aka micrograms per day), to the amount of PCB they inhaled at school and the amount they inhaled at home.  Recall that there are 1,000 ng in 1.0 ug.  Here is the summary table I ended up with:

Table 3 – Percentage of Daily PCB Exposure from Food and Air

The article’s conclusions were: 1) for these student receptors the overwhelming majority of their daily PCB dose comes from their diet; and 2) trying to reduce their daily PCB intake by reducing the concentration of PCBs in school air was a poor use of limited school resources because the proportion of the student’s daily PCB dose coming from air was too small to be of consequence.

What Happens When PCB Air Concentrations are even higher?

Let’s say you are growing a little skeptical about the potential health benefit of reducing PCBs in indoor air when the reduction is from 400 to 300 ng/m³.  But, what if the original indoor air PCB concentration is much higher than 400 ng/m³, what if it were 800ng/m³ (new Scenario 4) or 1,600 ng/m³ (new Scenario 5)?  Okay, take a look at Table 4:

Table 4 – Updated Sum of PCB Daily Exposures from Food and Air (in ug)

What I’ve done here is added the average daily dose of PCBs from food to the average daily dose of PCBs from air for our hypothetical elementary school students.  Scenarios 2 and 3 are the same as before, but new Scenarios 4 and 5 incorporate more extreme indoor air concentrations.  Scenario 5 is an unusually extreme indoor air concentration of PCBs.

Table 5 (below) uses the information just presented in Table 4 to identify what percentage of the student’s average daily PCB intake is coming from food and air with the new scenarios 4 and 5.

Table 5 – Percentage of Daily PCB Exposure from Food and Air

Table 5 shows that even when elementary school indoor air concentrations reach 1,600 ng/m³, only 27% of an elementary school student’s daily dose of PCBs would be from air (mostly from school air, but some from air at home too).

The Upside of Removing PCBs from Schools

Finally, Table 6 identifies the percentage reduction in average daily student PCB intake that can be achieved by reducing the indoor air concentration to the EPA Public Health Level of 300 ng/m³:

Table 6 – Percent of PCB Exposure Reduction from Reducing Indoor Air Levels

A Lot of Pain to Reduce PCB Air Concentrations, but is there Real Gain?

A conclusion that I did not highlight in the earlier article is that bringing indoor air concentrations down from 400 ng/m³ (a level well above the EPA public Health Level) to 300 ng/m³ will reduce a student’s average daily PCB intake by roughly 2%.  From a toxicological standpoint a 2% reduction cannot be expected to have any discernible health benefit, in fact it is almost certainly a difference too small to measure using even advanced methods.

      By reducing an indoor air concentration from 800 ng/m³ to 300 ng/m³ the total dose reduction for an average student would be just under 10%, again this difference in total PCB dose is too small to measure in a student population if the study were to be done.  Just think about the quality control acceptance criteria for PCB analysis (generally results that fall between 70% and 140% of the true value are considered accurate).  It is unlikely that a 10% difference in dose could be reliably detected.

      For scenario 5, the case with 1,600 ng/m³ PCB in air, it is harder to dismiss a potential 21% reduction in PCB dose as inconsequential.  That large a reduction in dose could in-fact be significant from a health perspective if the total dose received were on the “steep portion” of the dose-response curve.  The steep portion of a dose-response curve is the dose range where small changes in dose are most likely to have the biggest effect.  However, in the case of PCBs the steep part of the dose-response curve just begins at doses between 50 to 500 times greater than the doses we have been considering.  In other words, reducing PCB air concentrations in schools, even those as high as 1,600 ng/m³, are unlikely to produce any measurable health benefit because:

1. The PCB dose received from the average student diet is still much greater than the dose received from air; and
2. The total average daily dose of PCBs received by average elementary school children is much less than dose needed to produce detectable health effects.