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

The equation Gradient has developed to calculate worker intake of metals, such as lead, is based on a number of assumptions.

Not only do we have to accept the mean concentrations Gradient reports for these metal contaminants on the laundered shop towels, but we are also asked to assume that 13% of those contaminants will be transferred from the towel to the hand after the towel had been washed with soap and heat dried.

On top of this, as my last post described, Gradient makes the assumption that each time a laundered shop towel is used, the worker will place his hand to his mouth and 13% of the contaminants on the towel will be transferred to the mouth - intake.

Even if you accept Gradient's intake equation, it is difficult to accept the values they assigned for the assumptions used.

In my last post I detailed why the HTE of 13% Gradient uses is incorrect and an HTE is more appropriated since it involves calculating the HTE using both an adult soil consumption value and an adult hand.

Recalculating the intake values using an HTE of 6% still shows some metals to be above regulatory standards.  This is primarily due to the inappropriateness of calculating the HTE as a ratio of daily soil consumption to amount of soil found on both hands - as was discussed in my last post.

There is a more appropriate way to go about figuring out a hand to mouth transfer, which once again brings up:

Rule number 5: Always make sure the model and equations reflect reality.

Here is how CalEPA looks at lead intake from direct hand to mouth contact, which is the model Gradient should have used to calculate the intake of metals - such as lead - from laundered shop towels.

Source Page 6

Gradient instead calculates a lead intake over a worker's entire working lifetime of 40 years, whereas CalEPA calculates it on a single contact performed i number of times (events).  According to CalEPA:

There can be multiple hand-to-mouth contacts during the use of a given consumer product.  Thus the total direct lead intake via the use of a given consumer product will be the sum of intake from each contact i during product use.

CalEPA modifies the equation above as follows:

Source Page 6

Let's look at how CalEPA calculates the values for these parameters in their equation:

Source Page 11

                                     Surface area (SAD)

Source Page 12

                                    Contact frequency (λD) = Frequency
                         
                                    Exposure Duration (t) = Time


Lhand-D can then be calculated as follows:

Note: To keep consistent with other values used (see below), the surface of the front of the hand will be calculated as 840 (total surface area of both hands) * 0.5 (for one hand) * 0.5 (for the front of the hand).  Thus, the surface area for the front of one hand will be: 210 cm2.  In my previous "fun with graph paper" post, I estimated the surface area to be 188 cm2.

If Lhand-D is:

The lead loading on the part of the hand touching the mouth (not the loading of the whole hand), in units of weight per surface area (e.g., mass of lead per surface area of the fingertip, μg/cm2).

Assuming that 13% of the 75% lead load on the laundered shop towel is transferred to the hand:

• 0.00127 x 2268 x 0.75 x 0.13 = 0.28 mg
• 0.28 mg / 210 cm2 = 0.0013 mg/cm2 or 1.3 ug/cm2

 The "part of the hand touching the mouth" or SAD, is calculated as follows: 

Assumed for workers in occupational settings that the surface area of the hands contributing to the hand-to-mouth exposure pathway was 5% of the palmar surface of the hand, or 10 cm2.

Here is what I found in an earlier version of this CalEPA Lead document:

Source 2008

For this post, I will use the adult male SAD of 19 cm2.

Fdirect will be 50% (as per CalEPA)

Contact frequency (λD) will be 1.5 towels per hour (based on 12 laundered shop towels per 8 hour shift)

Exposure Duration (t) will be an 8 hour work shift.

For an exposure period of one day, lead intake during one work day (12 towels in 8 hours) - using CalEPA's equation - can be calculated as follows:

• Intake = 0.0013 mg/cm2 x 19 cm2 x 0.5 x 1.5/hour x 8 hours =  0.148 mg per work day
• 0.148 mg per day = 0.148 / 70 kg =  0.0022 mg/kg Intake or 2.2E-03

That's how CalEPA would calculate the intake based on a mean lead concentration of 100 mg and a towel to towel transfer efficiency of 13% - which are the same values Gradient uses in their equation.

Here is how the CalEPA method and equation stacks up against the Gradient equation and assumptions for the metals they reported with concentration exceedance ratios.

CalEPA Formula source

Notice how the CalEPA's equation produces an intake very similar to the values Gradient calculated for cancer intake.  Interesting.

Now lets look at the CalEPA method using an HTE of 6% (see post):

Notice how the CalEPA intake is very similar to the intake values obtained using Gradient's equation with an HTE of 6%.  Interesting.

The question you should now ask is: What equation more accurately estimates the intake if the assumptions used are valid?

I say we go with the CalEPA equation.  Now all we need to focus on will be the assumptions regarding the average load on the towel, the towel transfer efficiency, and the number of laundered shop towels used per day by a worker.  Oh yeah, we will also need to look at the respective "toxicity criteria" these intakes were compared to and how Gradient went about "evaluating the magnitude of the exceedances."

Next Post: Laundered Shop Towels: 10 - When is an average not average?


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