How do PCBs get into People?

 It is well known that all people have some amount of polychlorinated biphenyls (PCBs) in their body.  Now the total amount of PCBs we have in us is quite small and there is no real evidence that it is doing us any harm.  But, it is still a good question to ask: how did this happen?  Where did those PCBs come from?  More specifically, what types of exposures caused each of us to acquire our own personal cache of PCBs? 

 When you think the question through, you realize there are really just three possibilities:

 1. We were born with them, in other words we received them from our mothers during gestation in her womb;

2. We absorb PCBs that occur in small amounts from our food; and/or

3. We breathe air that contains small amounts of PCBs and retain them in our bodies.

 Which one of these is the best explanation?  Let’s see what we can figure out.  By the way, in the discussion that follows I am focusing on mean (average) consumers of food and breathers of air.  There are certainly people who for portions of their lives may differ significantly from these means, but most of the population will fall close to these values.

 Were we Born with PCBs?

 It was long assumed that a fetus was protected from drugs and other chemicals present in the maternal blood supply by the action of the placenta and its semi-independent circulatory system.  However, the thalidomide disaster proved that assumption to be false.  From human studies we know that the PCB concentration of umbilical cord blood has roughly half the PCB content of maternal blood.  So while the amount of PCB reaching the fetus from the mother is moderated, it is not eliminated.

 Still, we also know that young children generally have low to non-detectable concentrations of PCBs in their blood.  This may reflect a more rapid rate of PCB excretion for children, or it may be that the total body burden of PCBs received from the mother is small.  So while some portion of an adult’s PCB body burden may have been received from her/his mother during fetal development, the role of maternal PCB transfer to her infant appears to be small and transient.

 PCBs from Food

 It has been known for some time that PCBs are present throughout the human food supply.  The concentration of PCBs in food has been declining for decades and currently ranges from 2-6 micrograms of PCBs per kilogram of food (ug/kg) for fruits, vegetables and grains to 10-50+ ug/kg for chicken, meat, oils, butter and fish.

 To estimate the average daily PCB consumption from food, we need to know how much of the different types of food the average person eats, and the average PCB concentration in that food.  To keep things simple, I am assuming 100% of the PCBs ingested with food are absorbed into the body; a reasonable assumption in most cases.

 Using the USDA data on the average American food consumption and a mid-point in the range of PCB concentrations by food type I calculate that the average American consumes about 15.7 micrograms of PCBs per day in their food.  For comparison a person with a vegan diet (consuming no dairy or meat), the total PCB intake from food would be about 8.9 micrograms per day. 

 For the scenarios considered in the following section, I’ve reduced the average American dietary intake by 25% because the receptor in the exposure scenarios is an elementary school child (1,500 calories/day).  This results in a corresponding 25% reduction in PCB intake (11.8 ug/day for regular diet; 6.68 ug/day for a vegan diet).  If you would like to see my spreadsheet for these calcs please send me an email and I will forward them.

 PCBs from Air

 To estimate the average exposure to PCBs in air we need to know the breathing rate for the average elementary school student and the average PCB concentration in the air they breathe.  Table 6-1 of EPA’s Exposure Factor Handbook is a good source of information on breathing rate.  In the interest of simplicity, I am going to use a breathing rate of 12 cubic meters of air per day (m3/day) for the student. 

 This would be low for an adult and about right for a child.   Someone who is physically exerting themselves will have a much higher breathing rate while the activity lasts, but I am more interested in long-term average exposures than in peak short-term exposures.

 PCB background concentrations in outdoor air can be as low as 0.05 ng/m3 in remote areas and as high as 10 ng/m3 in outdoor urban environments (note that ng/m3 means nanograms per cubic meter; also note that 1,000 ng = 1.0 ug) .  Published data on indoor air PCB concentrations in cases where there is no PCB source material in the building is hard to find. 

 For the purpose of this post, I’m considering three theoretical air exposure scenarios:  

1) a student who lives in a home with 20 ng/m3 PCB in air who attends school in a building 32.5 hours/ week, 36 weeks/year with 100ng/m3 PCB;

2) a student who lives in a home with 20 ng/m3 PCBs in air who spends 32.5 hours per week 36 weeks/year in a building with 300 ng/m3; and

3) a student who lives in a home with 20 ng/m3 and who spends 32.5 hours/week, 36 weeks/year in a building with 400 ng/m3 PCBs.

 The indoor air concentration of 300 ng/m3 corresponds to the USEPA Public Health Level for elementary school age children.  The 400 ng/m3 indoor air concentration corresponds to a level 33% greater than the USEPA Public Health Level for elementary school age children.  Once again, if you’d like to see the spreadsheets, send me an email and I will pass them along.  For simplicity I’m assuming these individuals spend no time at all outside.  The following table shows the results.

Table 1 – Exposures to PCBs in Air

Scenario

Annual PCB Air Exposure (ug)

Average Daily PCB Air Exposure (ug)

1

153

0.42

2

343

0.94

3

438

1.20

Total Daily PCB Exposure

The sum the PCB exposures from eating and breathing are shown in Table 2:

Table 2 – Sum of PCB Daily Exposures from Food and Air (in ug)

Scenario

[PCBs]Food

[PCBs]Air

[PCBs]Total

1

11.8

0.42

12.2

2

11.8

0.94

12.7

3

11.8

1.20

13.0

Finally in Table 3 let’s consider what percentage of daily PCB exposure comes from food and what percentage comes from air:

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

Scenario

% PCBs from Food

%PCBs from Air

Total % PCB Exposure

1

96.6%

3.4%

100%

2

92.6%

7.4%

100%

3

90.8%

9.2%

100%

From Table 3 it is apparent that the major exposures to PCBs for these student receptors occurs through the diet and only a relatively small proportion is due to inhalation of PCBs in air, unless the air concentrations are significantly greater than 400 ng/m3.  So food appears to be the major route by which people receive their PCB body burden.

Based on this analysis, efforts to reduce human PCB exposure by reducing PCB concentrations in air in a school setting are often fundamentally flawed because the percentage of total PCB dose received through air is not great enough to make a significant difference in total PCB exposure.  Efforts to reduce PCBs in schools through expensive remediation programs can often be particularly misguided during a period of tight educational budgets.

There is an important limitation to this analysis, it only considers indoor air PCB concentrations up to 400 ng/m3.  Now 400 ng/m3 is a concentration well above normal levels, but indoor air concentration can be this high, or even higher, in some situations.  In my next blog post I’ll take a look at PCBs at higher indoor air concentrations.