Centre for Environmental Modelling and Chemistry

Some words may be used almost interchangeably in common speech, however, in the scientific context specific meanings are sometimes assigned to allow careful distinction to be made. Here we provide "our" definitions of some key terms.

"Uncertainty" describes the range in a measured value that is ascribable to, for example, the limitation of an analytical technique. Uncertainty in a value entered into a model will propagate through the model equations to cause an uncertainty in the model results.
"Error" in a model result may be caused by the use of an invalid assumption or incorrect input.
"Variability" describes natural and expected non-homogeneity, such as in the organic carbon (OC) content of soil in southern Ontario.
"Difference" is used to describe disparate values such as between the average OC content in southern Ontario and the average OC content of the Sahara desert.
Reference: Webster, E., Mackay, D., Wania, F., Arnot, J., Gobas, F., Gouin, T., Hubbarde, J., Bonnell, M. 2005. Development and Application of Models of Chemical Fate in Canada: Modelling Guidance Document. Report to Environment Canada. CEMN Report No. 200501 Trent University, Peterborough, Ontario.

"hazard" "risk" Reference: Arnot, J.A., Mackay, D., Webster, E., Southwood, J. M. 2006. A screening level risk assessment model for chemical fate and effects in the environment. Environ. Sci. Technol. 40 (7): 2316 - 2323.

	Conventional Equation	Fugacity Equation
BCF, Bioconcentration Factor	CB/CW	ZB/ZW
	biota / water equilibrium is assumed as resulting from only respiration uptake i.e., no metabolism	since CB/CW = fBZB/fWZW and fB = fW
BAF, Bioaccumulation Factor	CB/CW	fBZB/fWZW
	biota / water equilibrium is not assumed, uptake from both food and respiration is considered	This is the more general case of CB/CW where fB is NOT equal to fW
BMF, Biomagnification Factor	CB/CF	fB/fF
	biota / food	assumes LB = LF CB/CF = fBZB/fFZF = (fBLBZO)/(fFLFZO) = fBLB/fFLF since ZB = LBZO where LB is the volume fraction of lipid in the biota and ZO is the fugacity capacity in octanol, a surrogate for lipid

Reference: Webster, E., Cowan-Ellsberry, C.E., McCarty, L. 2004. Putting science into PBT evaluations. Environ. Toxicol. Chem. 23: 2473-2482.