The Researcher's Perspective
Podcast Interview with Mary H. Ward

Views and opinions expressed in these podcasts are those of the interview
subjects and do not necessarily reflect the views, opinions, or policies
of EHP or of the National Institute of Environmental Health Sciences.

Narrator: EHP presents The Researcher's Perspective.

Ernie Hood: Welcome once again to The Researcher's Perspective. I'm your 
host, science writer Ernie Hood. On this episode of The Researcher's 
Perspective, we welcome Dr. Mary H. Ward.  Dr. Ward is a Senior Investigator in 
the Division of Cancer Epidemiology and Genetics at the National Cancer 
Institute in Bethesda, Maryland.  She is the lead author of a study published in 
the June 2009 edition of EHP titled "Residential Exposure to Polychlorinated 
Biphenyls and Organochlorine Pesticides and Risk of Childhood Leukemia."  Dr. 
Ward, welcome to The Researcher's Perspective

Dr. Ward: Thank you.

Ernie Hood: Would you please give a brief summary of the study and your 
findings?

Dr. Ward: Sure.  We conducted a population-based, case/control study of 
childhood leukemia in 35 counties in northern and central California during the 
years 2001 to 2006, and this is called The Northern California Childhood 
Leukemia Study.  It included 17 counties in the San Francisco Bay area and 18 
counties in the Central Valley, and the study included 184 cases of acute 
lymphocytic leukemia, who were under 7 years of age, and 212 birth certificate 
controls.  We collected carpet dust samples from the room where the child spent 
the most time before diagnosis and a similar date for controls, using a 
specialized vacuum.  Now the purpose of this was to use carpet dust as an 
exposure indicator to examine risk of childhood leukemia in relation to residential 
exposures to persistent organochlorine chemicals, and these included 
polychlorinated biphenyls that have been banned since the late 70s, and some 
persistent organochlorine insecticides, including DDT and chlordane.  

The result of our study was that we found that detecting one of these PCBs in the 
dust was associated with about a two-fold increased risk of acute lymphocytic 
leukemia, the most common form of childhood leukemia, whereas the other 
organochlorine chemicals, DDT, chlordane and other organochlorine pesticides, 
were not associated with an increased risk.  And what was interesting and a bit 
puzzling is that we did not see as strong an association among Hispanic children 
and children of other races besides non-Hispanic whites, so essentially our 
association was strongest among non-Hispanic whites.  Now this was in spite of 
a very similar distribution in exposure between these different racial/ethnic 
groups.  So our findings suggest that PCBs, which we know are considered as 
probable human carcinogens, that there may be an association with childhood 
leukemia, but it's not clear why this would only be in non-Hispanic whites and not 
other groups.  This is the first study to evaluate this for childhood leukemia.

Ernie Hood: Dr. Ward, you and your coauthors found that PCBs may represent "a 
previously unrecognized risk factor for childhood leukemia."  What led you to 
suspect that residential exposure to PCBs might significantly increase the risk of 
that particular disease?

Dr. Ward:  Well, there's a couple of reasons.  One is that prior studies of adult 
blood tumors, lymphomas in particular, had found an association between PCB 
exposures measured in blood or serum, as well as in house dust in one study.  
Previous studies had shown a relationship between levels of these compounds in 
homes and risk of adult non-Hodgkins lymphoma.  And that was also found in 
studies where they looked at serum levels in adults.  Also we know that these 
chemicals persist indoors in carpets, where they're protected from degradation 
by sunlight, moisture, and micro-organisms.  So even though these chemicals 
have been banned for now over 30 years, they persist in the indoor environment.  

Ernie Hood: Dr. Ward, given the fact that, as you say, PCBs have been banned 
for several decades now but are still present in our environment, what do you see 
as potentially the wider implications of the finding that you had in this study that 
PCBs are sometimes present in household dust and are causing children to be 
exposed?

Dr. Ward: Well, we know there's not as much known about sources of PCBs in 
the homes as there is say for other contaminants like lead, where we know that 
the source is mainly from lead in paint that was used in the past.  There's just 
now beginning to be a little bit of research in this area that shows PCBs were 
used in floor finishers back in the 50s and 60s, and there's a couple of studies 
now that have seemed to pinpoint homes with very high levels were related to 
recent refinishing of these floors that were determined to be likely to have used 
this floor finish that we know contained high levels of PCBs.  PCBs were also 
used in caulking in commercial buildings and possibly in homes.  They're also 
found in old fluorescent light fixtures, and certainly they were present in industrial 
sites, so if a home had been previously built where there was contamination 
outdoors from industrial sources, that could be a source.  But we really haven't Ð 
this isn't nearly as well studied as some other contaminants, and it's not really 
clear what the sources are.  We do know from our data that PCB levels were 
found at higher concentrations in older homes, in other words homes that were 
built before 1980, before these chemicals were banned.

Ernie Hood: Dr. Ward, the design of the study was very interesting.  As you 
mentioned, you analyzed the chemical contents of household carpet dust to 
estimate PCB exposure among children. Is household carpet dust itself a 
potentially significant source of exposures to harmful chemicals such as PCBs?  

Dr. Ward: Carpet dust can be a potential source of exposure, particularly for 
children, who spend most of their time indoors, young children who spend most 
of their time indoors and frequently put their hands in their mouths.  We also 
know for these persistent organochlorine compounds that diet is an important 
route of exposure as well.  

Ernie Hood: Dr. Ward, as you point out in the article, the results of this study are 
"highly suggestive."  What further research would you like to see to expand our 
knowledge about the potential role of exposures to PCBs in the etiology of 
childhood leukemia?

Dr. Ward:  Well, I think we need to do further studies.  Because childhood 
leukemia is so rare, that isn't such an easy thing to do, but in the Northern 
California Childhood Leukemia Study, case ascertainment is ongoing and 
hopefully in the future we'll have an opportunity to evaluate this in cases and 
controls identified in the future.  Also we'll look for opportunities to evaluate this 
hypothesis in other study populations.  And I think an important area of research 
is really to understand whether residential concentrations really are a good 
predictor of exposure in terms of serum levels of these chemicals, and what 
proportion of exposures come from house dust exposure for children, as 
opposed to other sources like diet, and breastfeeding from their mothers.  

Ernie Hood: Dr. Mary H. Ward, thank you so much for joining us on this edition of 
The Researcher's Perspective.

Dr. Ward: You're very welcome.

Ernie Hood: And thank you for listening to The Researchers Perspective!