So anybody else looked more closely at the Agency approach for residential 
SGV for cadmium and the model output?


The Agency has adopted AC1-18 for cadmium.  Because of the exposure model 
set-up, this has the counter-intuitive effect of actually raising the CLEA model 
output compared with just AC1-6; i.e. the longer you are get exposed, the 
safer it gets….

This is discussed on Page 7 the pdf of the SGV document:
 
“Young children tend to have the highest potential exposures to contaminants 
in soil; this combined with their lower body weights compared with adults 
means they are often at greatest risk of exposure exceeding the TDIoral or 
TDIinh (Environment Agency, 2009b). Averaging exposure over a lifetime 
typically results in the exposures for young children exceeding the relevant TDI, 
even though overall exposure over the lifetime does not (Environment Agency, 
2009b). In evaluating the impact of applying lifetime exposure to the derivation 
of the SGV for cadmium, the total average exposure for young children (from 
soil and non-soil sources) over age classes 1–6 has been estimated to be about 
two and a half times the TDIoral. This is not anticipated to be of significant 
toxicological concern (Environment Agency, 2009d). However, long-term 
exposure to levels in excess of either the TDIoral or the TDIinh might be 
associated with an increase in kidney disease in a proportion of those 
exposed.”

The key phrases are: 

“the total average exposure for young children (from soil and non-soil sources) 
over age classes 1–6 has been estimated to be about two and a half times the 
TDIoral.  This is not anticipated to be of significant toxicological concern 
(Environment Agency, 2009d).”

So the Agency accepts that the SGV based on the oral TDI appears not to be 
protective of AC1-6.  This is on the toxicological basis for the oral TDI, 
discussed in Section 4.11.2 of the Cadmium Tox Report:

“In each case, a critical concentration of cadmium in the renal cortex for the 
onset of the early stages of kidney disease has been established (or assumed) 
and the corresponding average daily intake that over 50 years or so would 
produce such a kidney burden has then been estimated.”

LQM have produced a residential GAC of 3 mg/kg, which is significantly lower 
than the Agency value of 10 mg/kg.  LQM appears to attribute the difference in 
output to the HCV for inhalation and have (I assume) used AC1-6.

However, if you run CLEA-1.04 with AC1-6 using the SGV inputs, the model 
outputs 5.2 mg/kg.  Which is pretty consistent with LQM, so it would appear 
that the main difference in outputs is due to the AC1-6 by LQM versus AC1-18 
by the Agency?


This brings me to my last concern.  I think the CLEA 1.04 model output for the 
Agency data using AC1-6 should actually be 4.6 mg/kg and not 5.2 mg/kg.

There appears to be an error in the goal seek? A manual check of the inverse 
calculation from the CLEA oral & inhalation outputs (5.45 & 29.7 mg/kg) gives.

GAC = 1/((1/5.45)+(1/29.7))= 4.6 mg/kg

A further check running the Agency data AC1-6 with MDIs = 0 yields 9.2 
mg/kg: as the Agency model is using the 50% rule for both oral and inhalation, 
then with MDI the model output should be 9.2 * 50% = 4.6 mg/kg??  (Ignore 
the % contribution Agency data as this relates to intake and NOT %ADE).

Anomaly confirmed by our own In-House CLEA Model which also gives 4.6 
mg/kg.

Have attached a zipped up CLEA spreadsheet with Cadmium data with & 
without MDIs for AC1-6.  Can some people confirm I’m being stupid on this 
point.  Is there some nuance I am missing in the final CLEA SAC calcs?


So there we have it.  I would be tempted for the residential cadmium GAC to 
opt for a value of 5 mg/kg (rounded up from 4.6 mg/kg or down from 5.2 
mg/kg).

This also saves all the faff of the whole AC1-18 input hassle for cadmium - 
keep it simple!  And 5 mg/kg seems a reasonable real world number for top soil 
and shouldn’t cause unnecessary issues on most sites.  And it is more 
consistent with the LQM value of 3 mg/kg.


And as an aside, residential without plant uptake for AC1-6 is c. 13 mg/kg.


Interested to hear what people think.


Chris Dainton