CHEMICAL PROFILES|Human Health Hazards

Chemical: DIAZINON
CAS Number: 333-41-5
U.S. EPA developed the Waste Minimization Prioritization Tool (WMPT) to help guide the national effort to reduce the quantity and toxicity of hazardous wastes. WMPT is a software system that provides relative rankings of more than 1,300 chemicals according to their environmental persistence, bioaccumulation potential, and human and ecological toxicity. The WMPT ranks chemicals separately according to their human health risks, their ecological health risks, and their overall environmental health risk.

WMPT human health scores indicate how a chemical compares with others in terms of its capacity to harm human health. The graphic shows where a chemical's human health score falls relative to all chemicals that have been ranked using the WMPT system, and indicates whether a chemical is more or less toxic than most chemicals. Chemicals that score at the far right end of the scale are significantly more hazardous(in the worst 5% according to this scoring system).

All chemicals scored by a system are placed in "bins" defined by percentiles (e.g., a chemical's score is in the least toxic 25% of chemicals scored by a system). The graphic illustrates which bin a chemical falls in according to each scoring system in Scorecard. Looking across these different systems, it is possible to identify chemicals that consistently score as high or low hazards, as well as chemicals that score high on some measures (such as human health hazards) but low on others (such as ecological hazards).

WMPT scores are based on measures of toxicity, persistence in the environment and bioaccumulation potential. The system has important limitations because it does not actually estimate human exposures to toxic chemicals, but relies on more readily available surrogate measures of exposure potential.

The WMPT scoring system assigns twice as much importance to a chemical's exposure potential as it does to its inherent human toxicity. A chemical's rank is driven by the scores assigned to its persistence and bioaccumulation potential, and substantial variations in toxicity potential may not be reflected in a chemical's human risk screening score.


Human Health Risk Screening Score (WMPT) = 6

The WMPT ranking system, although only a screening-level method, follows the same conceptual framework commonly used in standard risk assessment. Standard human health risk assessment methodology quantifies human health risk potential as a function of both human toxicity and human exposure potential:

Human Health Risk Potential = (Human Toxicity) * (Human Exposure Potential)

However, because the data used in the scoring calculations often varies by several orders of magnitude, the scoring scales are roughly approximated by a log scale, turning the multiplicative relationship into an additive relationship:

Human Health Risk Potential = (Human Toxicity) + (Human Exposure Potential)

The human toxicity factor evaluates adverse effects to human health from chronic exposures, and is calculated from subscores for cancer and noncancer effects. The WMPT ranking system assigns toxicity scores of 1 - 3 based on quantitative measures of a chemical's human toxicity and on qualitative considerations. For cancer effects, scores are assigned by comparing cancer potency factors to "fencelines" that divide the range of potency measures into high, medium and low toxicity groups, and include consideration of the weight of evidence indicating a chemical could cause cancer in humans. For noncancer effects, scores are assigned by comparing available risk assessment values (or experimental NOAEL/LOAELs) to "fencelines" that divide the range of potency measures into high, medium and low toxicity groups, and take into account data quality. In cases where the chemical has both cancer and noncancer effects, the highest of the two scores is used as the human toxicity factor.

Ideally, the human exposure factor would be based on monitored or estimated environmental concentrations associated with various chemical use or release scenarios, as well as data on the resulting exposures experienced by human populations. Since such information is rarely available, WMPT focuses instead on exposure potential, as indicated by two more widely available surrogate measures: persistence and bioaccumulation potential.

The WMPT ranking system assigns exposure potential scores of 2-6 by adding sub-scores given to surrogate measures of a chemical's ability to persist and bioaccumulate in aquatic environments. Measures of environmental persistence utilized by WMPT are ordered by data quality and include estimated biodegradation time in water and soil and estimated hydrolysis half-life. Measures of bioaccumulation utilized by WMPT are ordered by data quality and are measured or estimated bioaccumulation and bioconcentration factors. This reflects a significant change from the WMPT Beta Test Version 1.0, in which bioaccumulation potential scores for most organic chemicals were based on estimated values of the n-octanol-water partition coefficient (Kow). Scores are assigned based on "fencelines" that divide the range of each of these measures into high, medium and low exposure potential groups.

Finally, the human toxicity factor is added to the human exposure potential factor to calculate the chemical's overall human health risk potential. In sum:

Human Health Risk Potential (3-9) = Human Toxicity (1-3) + Human Exposure Potential (2-6)

WMPT: US EPA, Office of Solid Waste and Office of Pollution Prevention and Toxics. Waste Minimization Prioritization Tool: Background Document For The Tier III PBT Chemical List. Appendix A: WMPT Summary Spreadsheet. EPA, Washington, DC. July, 2000.