Toxic gas categories and TLV ranges: A gas with a permissible safe limit (TLV) in the range of approximately 100–1000 ppm would best be classified as which toxicity category?

Introduction / Context:
Threshold limit values (TLVs) provide guidance for safe airborne concentrations of chemicals in occupational settings. In many environmental engineering texts, qualitative toxicity categories are loosely related to order-of-magnitude TLV ranges to support risk prioritization. This question asks which category matches a TLV on the order of 100–1000 ppm.

Given Data / Assumptions:

• TLV range considered: about 10^2 to 10^3 ppm for gases.
• Relative classification scheme: lower TLV implies higher toxicity.
• Context aligns with typical textbook categories (extremely, highly, moderately, etc.).

Concept / Approach:
Chemicals with extremely low TLVs (for example, less than 1 ppm) are considered extremely toxic (e.g., methyl isocyanate). As TLV values rise into tens and hundreds of ppm, the relative toxicity category becomes moderate because higher ambient concentrations are tolerated before adverse effects occur. Values above 1000 ppm might be considered low acute toxicity under short exposure for some gases, but 100–1000 ppm generally aligns with moderate toxicity in many tabulations used for quick hazard screening.

Step-by-Step Solution:
Recognize the inverse relation: lower TLV → higher toxicity.Compare the provided band (100–1000 ppm) with qualitative categories.Map to “moderately” toxic based on common instructional ranges.

Verification / Alternative check:
Reference tables in industrial hygiene training commonly place very hazardous gases in the sub-ppm to single-digit ppm range, while moderate hazards often occupy the 100+ ppm region for TLVs or short-term exposure limits, supporting the selected category.

Why Other Options Are Wrong:
Highly / Extremely / Very extremely: These denote much lower permissible concentrations (often sub-ppm to tens of ppm), inconsistent with 100–1000 ppm.

Common Pitfalls:

• Treating TLV categories as absolute; they are heuristic and can vary by source.
• Ignoring exposure duration differences (TWA vs. STEL).

Final Answer:
moderately