Inhalation cancer potency factor (slope factor) is given by CalEPA as 510 (mg/kg-day)-1 (link here: https://oehha.ca.gov/chemicals/chromium-hexavalent). This is expressed in units of proportion (of a population) affected per mg of substance/kg body weight-day. I would recommend that you review the information on the US EPA website, prior to using the value given by CalEPA:

• https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=144
• http://cfpub.epa.gov/ncea/iris/iris_documents/documents/subst/0144_summary.pdf#nameddest=cancerinhal.

I believe that you may have made a mistake in your calculations in order to arrive at a risk >1. Acceptable levels of lifetime cancer risk are typically between 1x10-4 and 1x10-5 depending on circumstances i.e. between 1in 100,000 and 1 in 10,000 people would get a cancer at a given exposure level. Is it possible that you have used the wrong units somewhere?

Also, please note that these methods of calculation are designed to be protective of public health i.e. they are not considered to be accurate, but rather conservative. I wrote something about why this matters in the following paper (although it is documented by the US EPA in great detail elsewhere, and I realise you may well be aware of this already).

Article Assessing the human health impacts of exposure to disinfecti...

(Green Open Access version archieved here: https://zenodo.org/record/896153)

I cannot comment on the CEPA approach but I can comment on calculating cancer risks from exposure to Cr(VI).

Are you familiar with the regulatory context of Cr(VI) compounds in the European Economic Area (EEA)? These substances are subject to Authorisation under EU's REACH Regulation (EC) No 1907/2006. I will not go into the detail of REACH Authorisation but please be aware that in the context of it, the Risk Assessment Committee (RAC) of the European Chemicals Agency (ECHA, based in Helsinki) developed a dose-response relationship (also known as exposure-risk relationship) for Cr(VI). This provides the excess lifetime (lung/intestinal) cancer mortality risk for workers and consumers due to inhalation and oral exposure. Perhaps the analysis therein would be of help to you?

This allows the translation of lifetime exposure (expressed in ug/m3 (inhalation) or ug/kg bw/day (oral)) into the risk of dead from cancer (lifetime: 40 year working life (8h/day, 5 days/week) for workers and 70 years (24h/day, every day) for consumers). Here is the link:

https://echa.europa.eu/documents/10162/13579/rac_carcinogenicity_dose_response_crvi_en.pdf/facc881f-cf3e-40ac-8339-c9d9c1832c32

With regard to reprotoxic effects, because these effects are assumed to have a threshold, the RAC has issued a document outlining the Derived No Effect Levels (DNELs) for fertility and developmental toxicity effects following exposure to Cr(VI) via inhalation/oral/dermal routes (note though that the carcinogenic effects of Cr(VI) are much more potent than its reprotoxic effects). Here is the link:

https://echa.europa.eu/documents/10162/21961120/rac_35_09_1_c_dnel_cr-vi-_en.pdf/8964d39c-d94e-4abc-8c8e-4e2866041fc6

Let me know if you need any further help on this.

The calculated risk can not be greater than 1. I think you may have a mistake in the unit of the different terms in the CDI formula. Check the units and recalculate.