Hello!  Perhaps the following links to the U. S. EPA Exposure Factors Handbook 2011 and a recent research article with additional reference therein will be of some assistance to you:

1)  Exposure Factors Handbook 2011 (U. S. EPA)

http://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=236252

2)  Human health risk assessment of heavy metals in soil-vegetable system: A multi-medium analysis (LIU et al., 2013)

http://sewagesludgeactionnetwork.com/files/Human-Health-Risk-Heavy-Metals-in-Vegetables.pdf

Thank  you  very much C. Mei-Ling Hamilton, 

You are quite welcome!

Francois, you may find this helpful also.

Thank you clement, very interesting documents, I found the formula, what I was looking for !