Safety siting guideline: for a hazardous chemical plant producing a substance with TLV (threshold limit value) less than 1000 ppm, what is a reasonable safe distance of nearby habitation (in km)?

Introduction / Context:
Siting and layout decisions for hazardous chemical plants aim to reduce offsite risk to communities. A common early-stage screening metric is the separation distance from sensitive receptors (residential areas, schools) based on the material’s inherent toxicity (e.g., TLV) and credible release scenarios. While detailed quantitative risk assessment (QRA) refines this, rule-of-thumb buffers help at concept stage.

Given Data / Assumptions:

• Substance has TLV < 1000 ppm (i.e., relatively toxic).
• Flat terrain, typical meteorology; no complex topographical channeling.
• Goal: preliminary planning value rather than final regulatory siting requirement.

Concept / Approach:
Lower TLV implies health effects at lower airborne concentrations, so larger buffers are prudent to attenuate dispersion peaks from accidental releases. Generic siting practice often suggests several kilometers of stand-off distance for toxic inventories pending a full dispersion and risk study. For TLV below 1000 ppm, a ~5 km separation provides a reasonable interim safeguard, balancing practicality and precaution.

Step-by-Step Solution:

Verification / Alternative check:
Preliminary dispersion estimates (e.g., Gaussian models) for moderate inventories often show concentration decay to fractions of TLV over a few kilometers, supporting the 5 km planning distance before design-level studies.

Why Other Options Are Wrong:

• 1 km: commonly inadequate for toxic clouds under stable night conditions.
• 15 km or 25 km: excessively conservative for preliminary layout, typically reserved for very large inventories or worst-case toxicants.

Common Pitfalls:
Confusing planning buffers with regulatory requirements; failing to update the buffer after QRA; ignoring terrain, weather extremes, and emergency response capabilities.

Final Answer:
5