Laundered Shop Towels 8: A child's hand is not an adult's hand

That title is one of those "duh!" types of statements.

So if that's true, why did Gradient base the hand to mouth efficiency (HTE) on studies involving 1-6 year olds?

Not that there is anything wrong with that, but in this case, we are dealing with adult workers and assumptions should have been made using adult data - that was readily available to them.

Gradient states in their 2003 study the following:

Gradient used the median skin surface area data specific to a 1- to 6-year-old child and applied the soil AF derived from Roels to estimate the average mass of soil on the hands for a 1- to 6-year-old child, which is approximately 145 mg for both hands.

A median soil ingestion rate of 38 mg/day for children ages 1 to 6 years was calculated based on a soil ingestion study conducted in Amherst, Massachusetts.

This soil ingestion rate was divided by the hand soil-loading estimate for a child resident (approximately 145 mg on both hands), for a daily HTE value of approximately 0.26 hand loads per day.

In this report, we used half of 0.26 as the HTE value for adults, or 0.13. 

Using a smaller HTE for adults as compared to children is further supported by the United States Environmental Protection Agency's (USEPA) soil ingestion rates:  their recommended mean soil ingestion rate for adults is exactly one-half of the value for children less than 6 years of age (USEPA, 1997a)

That HTE value of 13% was based on dividing the amount of soil a child consumes in a day by the amount of soil both hands of a child can hold (soil-loading).  Read my previous post on this for more information.

That amount, 26%, was then divided in half to represent the HTE for an adult, 13%.

Sounds good until you think about it a little more closely.  See it?  Yeah, you can fly a Russian Antonov An-225 through this one.

According to Gradient, the "145 mg for both hands" was calculated as follows:

Gradient then divided the average amount of soil adhering to the hands by the “available” skin surface of the hands for the average age of the children included in the Roels study (i.e., 11-yearolds) to generate a soil adherence factor (AF) of 1.1 mg/cm2 for both boys and girls.  

The skin surface area of the hands available for contact with soil is assumed to be approximately one-third of the total surface area of both hands.

If that assumption holds true, why didn't Gradient use "one-third of the total surface area of both hands" for an adult?

To get the HTE of 13% the "median soil ingestion rate of 38 mg/day for children ages 1 to 6 years" was divided by "145 mg for both hands."

If we assume (according to the EPA) that the "mean soil ingestion rate for adults is exactly one-half of the value for children less than 6 years of age," wouldn't it have been more appropriate to take one-half of 38 mg/day - 19 mg/day - and divide that by  "one-third of the total surface area of both hands" for an adult?

If that assumptions for a child holds true, this would have been a more appropriate - or scientifically sound - method to calculate the HTE for an adult worker.

You can ask Gradient why they did not use this method to calculate their HTE.  Even more peculiar is why they did not use an established calculation to estimate hand to mouth intake for an adult.  A little bit of Google searching brings up this document from the CalEPA:

Guideline for Hand-to-Mouth Transfer of Lead through Exposure to Consumer Products

Here is what CalEPA says on page 12:

The U.S. EPA Exposure Handbook provides representative hand surface area values for both adults and children in Chapter 6, General Factors for Dermal Route.  Detailed data distributions of hand surface area (mean, standard deviation and percentile distributions) by gender and age are provided in Tables 6-2 to 6-8 (U.S. EPA, 1997). 

Gosh...I wonder what that source is?

U.S. Environmental Protection Agency (U.S. EPA, 1997). Exposure Factors Handbook.  National Center for Environmental Assessment, Office of Research and Development, Washington, DC, EPA/600/p-95/002F a-c.

That sounds familiar...I wonder where I saw that source mentioned before?  Oh, yeah, it was referenced in the 2003 Gradient study on laundered shop towels:

Source: 2003 Gradient Study

Why's that important?  Well in that EPA handbook is data that Gradient should have used.  Here is what CalEPA goes on to say:

From the U.S. EPA Exposure Handbook, the representative value of the surface area of both hands in adults is 750 cm2 for women and 840 cm2 for men. 

I wonder what the HTE would be if we used the values assigned to adults?  Let's see:

"The skin surface area of the hands available for contact with soil is assumed to be approximately one-third of the total surface area of both hands."  So if we multiply 840 by 0.33 we get 274 cm2

"Gradient then divided the average amount of soil adhering to the hands by the “available” skin surface of the hands...to generate a soil adherence factor (AF) of 1.1 mg/cm2 for both boys and girls."  So if we multiply 274 by 1.1 we get 301 mg on both hands, this is the "soil loading" estimate.

If the adult "mean soil ingestion rate for adults is exactly one-half of the value for children less than 6 years of age," we would take 38 mg/day and divided it by 2, which would give us 19 mg/day. 

 "This soil ingestion rate was divided by the hand soil-loading estimate...for a daily HTE value..." So, if we divide 19 by 301 we would get an adult male HTE of  6%.

As I pointed out in a previous post, this method of calculating an HTE is flawed because it assumes all the soil ingested in a day comes solely from the hands.

Still, though, if Gradient was going to calculate an HTE based on this method, it would have made more sense to use adult values, which would have been calculated as 6%.

Does an HTE of 6% instead of 13% affect their findings?

Recalculation - Table 8 of 2011 Study

I had to tweak the "Average Load" values in Table 8 upwards to get the same intake values they calculated.  The values in blue represent the intakes one would see if a 6% HTE was used.  At a 6% HTE, the Exceedance Ratios in Table 8A are changed as follows:

You can see that there are still exceedances, but adjusting just one variable, the HTE, changes the results considerably.  Now, consider the other parameters "assumed" to be correct by Gradient.  Every value that is used that is higher than would actually be in reality (mean. maximum, towel transfer efficiency, number of rags used), exaggerates the "exceedance ratios" Gradient reports.

Yeah...but look at lead!  It's still 273 times higher than CalEPA's MADL.... and five times higher for cancer.  Explain that!

Those exceedance values are only applicable if you accept Gradient's HTE..  Remember, Gradient calculated that percentage based on ALL of the soil consumed in a day coming from the hands.  13% or 6% is based on that premise, which is not the sole mechanism for soil intake into the child or adult. (see post).

A better way to calculate how much lead (metal) would be transferred from the hand to the mouth would have been to use a more plausible calculation....like the one that CalEPA has developed.

Next Post: Laundered Shop Towels 9: CalEPA's Lead Intake from Direct Hand-to-Mouth Contact


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