Human Health Risk Assessment (HHRA)
Quantifying the Risk to People from Chemical, Physical, and Biological Hazards
A Human Health Risk Assessment is the formal, science-based process of evaluating whether exposure to hazardous substances in a workplace, community, or contaminated environment poses an unacceptable risk to human health. Unlike a simple hazard identification or a qualitative risk matrix, an HHRA applies quantitative toxicological data, exposure modelling, and dose-response analysis to determine the probability and severity of adverse health effects for specific populations — workers, residents, site visitors, or vulnerable groups such as children and pregnant women.
The Four-Step HHRA Framework
Step 1: Hazard Identification
Determines which substances are present, their toxicological properties, and the types of health effects they can cause. Establishes the conceptual site model (CSM) — the source-pathway-receptor linkages that define how contaminants reach people.
Step 2: Dose-Response Assessment
Quantifies the relationship between exposure magnitude and health effect probability. For threshold substances: reference doses and tolerable daily intakes. For non-threshold carcinogens: slope factors and unit risk values from WHO, US EPA IRIS, and enHealth.
Step 3: Exposure Assessment
Estimates magnitude, frequency, duration, and route of human exposure for each receptor population. Calculates chronic daily intake via inhalation, ingestion, and dermal pathways using published exposure factors and site-specific data.
Step 4: Risk Characterisation
Integrates all steps into quantitative risk estimates. Hazard quotients for threshold substances (acceptable if HQ < 1.0). Incremental lifetime cancer risk for carcinogens (acceptable if ILCR < 10⁻⁵). Cumulative risk assessment for multi-substance exposures.
HHRA in the Workplace Context
A workplace HHRA goes beyond comparing monitoring results to the WEL for a single substance. It considers the cumulative effect of exposure to multiple substances, the contribution of dermal and ingestion pathways, the impact of shift patterns on chronic dose, and the specific vulnerabilities of worker subpopulations. The December 2026 WEL transition introduces over 30 NTGCs — substances for which no Australian limit previously existed. For workplaces using these substances, an HHRA may be the only way to establish a defensible exposure management strategy.
Regulatory and Legal Context
In NSW, contaminated land HHRAs are conducted within the CLM Act 1997 and NEPM 2013 framework. The NSW EPA and local councils require HHRAs for contaminated sites being rezoned or redeveloped for sensitive land uses. For workplace health risks, the HHRA supports the PCBU's duty of care under the WHS Act 2011. The legal defensibility of an HHRA depends on assessor competence, data quality, assumption transparency, and methodological rigour.
Commission an HHRA
Whether you need a contaminated site HHRA for a property transaction, a workplace HHRA for cumulative chemical exposure, or a Tier 2 site-specific risk assessment, OHConnect connects you with qualified risk assessors.