Screening-level risk comparisons: The Human Toxicity Potential:
The Human Toxicity Potential (HTP) is an indicator of the danger posed by a
chemical's release to air or surface water. It was developed to compare
emissions in life-cycle assessment (LCA) and public emissions inventories,
such as the US Toxic Release Inventory (TRI) and the European Pollution
Emissions Register (EPER). HTP contains two elements: one is the toxicity of
the chemical, represented by the unit risk factor (for carcinogens) or the
safe dose (RfD, for non-carcinogenic effects). The second is the Potential
Dose, the intake of the pollutant by an individual living in a certain model
We have now published HTP values for 330 chemicals . These HTP values
were calculated using the CalTOX model, a multimedia fate and exposure model
developed originally to support the state of California's risk assessments.
It considers 7 different compartments and 23 possible exposure pathways.
The potential dose takes into account a chemical's persistence as well as
its accessibility for human intake (e.g., through bioaccumulation or
A careful analysis of the model and its uncertainties, especially
considering the wide range of chemical properties , has lead to a number
of model improvements. For example, the modeling of exposure through plants
had to be changed because the dominant modeling approach exposure modeling,
which was originally used in CalTOX, did not appropriately account for the
behavior of about 10% of the chemicals. In addition, we found that steady
state assumptions would underestimate inhalation exposure of 25% of our
chemicals, those that are hydrophilic. We hence developed a new approach
that allows us to investigate intermittent conditions in multimedia models
 and found that, given other uncertainties, for HTP it was sufficient to
simply combine rain and no-rain conditions .
To complement the human toxicity potential, we have also developed estimates
of the spatial range of these chemicals (over land) . These estimates
can be used to investigate how concentrated or widely distributed exposure
occurs. This allows evaluations of the number of people exposed to a
particular emissions, given the population distribution around a release
site. In addition it allows evaluation of the potential long range or
trans-boundary transport of pollutants.
To ensure the high quality of our work, we have also developed a strategy
for evaluating model performance of CalTOX .
HTP data are available from Edgar Hertwich's web site. All journal articles
are available online but may require a special login.
1. Hertwich EG, Mateles SF, Pease WS, McKone TE. 2001. Human Toxicity
Potentials for Life Cycle Assessment and Toxics Release Inventory Risk
Screening. Environ. Toxicol. Chem. 20(4):928-939.
2. Hertwich EG, McKone TE, Pease WS. 2000. A systematic uncertainty
analysis of an evaluative fate and exposure model. Risk Anal. 20(4):437-452.
3. Hertwich EG. 2001. Intermittent Rainfall in Dynamic Multimedia Fate
Modeling. Environ. Sci. Technol. 35(5):936-940.
4. Hertwich EG, McKone TE. 2000. The Spatial Scale of Pollutants in
Multimedia Models and its Implications for the Potential Dose. Environ. Sci.
5. McKone TE, Hertwich EG. 2001. The Human Toxicity Potential and a
Strategy for Evaluating Model Performance in Life Cycle Impact Assessment.
Int. J. Life Cycle Assess. 6(2):106-109.
Edgar Hertwich, PhD
LCA Laboratory, Inst. for Product Design Engineering
Norwegian University of Science and Technology
Kolbjørn Hejes vei 2b
7491 Trondheim, Norway
Tel. +47-73-55 0634
Fax +47-73-59 0110