Toxic Gas Dispersion: Understanding ERPG Levels

When containment fails, fire and explosion hazards are usually visually obvious. However, toxic gas releases are often the "silent killers" in process safety. Depending on the substance, a toxic plume can be entirely invisible and odorless until it reaches lethal concentrations.
Under the South African Major Hazard Installation (MHI) Regulations, predicting where these gases will go and how they will affect people is a mandatory requirement. This is achieved through Consequence Modeling, specifically toxic dispersion modeling. But to understand the model's output, we first must understand the benchmarks of human survivability: ERPG levels.

What are ERPG Levels?

ERPG stands for Emergency Response Planning Guidelines. Developed by the American Industrial Hygiene Association (AIHA), ERPGs are the global standard used by process safety engineers to estimate the health effects of a short-term toxic exposure.
They are divided into three tiers, based on a 1-hour exposure time:

ERPG-1: The Nuisance Threshold

This is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour experiencing only mild, transient adverse health effects or perceiving a clearly defined, objectionable odor.

• Real-world translation: "I can smell it, my eyes might water a bit, but I can still safely evacuate without lasting harm."

ERPG-2: The Escape Threshold

This is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action.

• Real-world translation: "This is extremely uncomfortable. It burns, and I might need medical attention later, but I am still physically capable of escaping the area."
• Regulatory Importance: In MHI assessments (guided by SANS 1461), the ERPG-2 contour is frequently used as a critical boundary for emergency planning. If members of the public (outside your boundary wall) are exposed to ERPG-2 levels, your facility will almost certainly be classified as an MHI.

ERPG-3: The Survivability Threshold

This is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects.

• Real-world translation: "Exposure at this level is lethal or causes permanent, debilitating damage."

Factors Affecting Toxic Gas Dispersion

You cannot simply draw a circle around a leaking tank and call it the hazard zone. Toxic dispersion is highly dynamic and depends on several critical variables:

1. Vapor Density (Heavier or Lighter than Air):

• Gases like Ammonia ($NH_3$) are lighter than air and will tend to rise and dissipate faster.
• Gases like Chlorine ($Cl_2$) are significantly heavier than air. They will "slump," hugging the ground, filling trenches, and traveling far downwind before diluting.

1. Weather and Atmospheric Stability:

• A windy day disperses toxic gases quickly.
• A calm, clear night (Pasquill Stability Class F) creates a temperature inversion. The gas is trapped near the ground and barely dilutes. In South Africa's Highveld winters, these stable night-time conditions often produce the worst-case consequence footprints in our Quantitative Risk Assessments (QRA).

1. Release Rate and Duration:

• An instantaneous release (a vessel rupture) creates a single, moving toxic "puff."
• A continuous release (a broken pipe) creates an ongoing toxic "plume." Your emergency isolation valves dictate how long this plume lasts.

Why ERPGs Matter for Your MHI Assessment

If your facility stores toxic substances—whether it's chlorine at a municipal water treatment plant or ammonia at a cold storage facility—you are required to assess the risk to both your employees and the neighboring public.
If your dispersion modeling indicates that an ERPG-2 contour extends beyond your site boundary, you have a legal obligation under the MHI Regulations to:

• Register as a Major Hazard Installation.
• Inform local authorities and neighboring properties.
• Develop a robust Emergency Response Plan (ERP) in collaboration with local emergency services.

Beyond Compliance: Protecting Lives

At MMRisk, we don't just calculate your ERPG contours to check a compliance box. Our SANAS-accredited engineers use advanced software like PHAST to visualize exactly how toxic gases will behave on your specific terrain under local weather conditions.
We use these insights to help you design better barrier systems, optimize your Hazard Identification (HAZOP) studies, and safeguard your people.
Is your facility storing toxic gases? Don't wait for the 2026 MHI deadlines. Contact MMRisk today to schedule a screening assessment and discover exactly where your ERPG contours lie.