Air quality standards – Threshold Limit Value (TLV) for sulfur dioxide: What is the commonly cited safe concentration (TLV) for SO2 in workplace or ambient air, expressed in parts per million (ppm)?

Introduction / Context:
Threshold Limit Values (TLVs) and similar guideline values set safe upper bounds for pollutant concentrations to protect health. Sulfur dioxide (SO2) is a pungent, water-soluble gas arising from combustion of sulfur-bearing fuels and certain industrial processes.

Given Data / Assumptions:

• A single-value multiple-choice recall of a widely taught TLV for SO2.
• Older exam and textbook conventions often reference a 5 ppm ceiling/short-term level in simple questions.

Concept / Approach:
While modern occupational limits vary by jurisdiction and can be lower, traditional engineering exam keys frequently use 5 ppm as the indicative safe upper bound for short exposures. Extremely high values like hundreds or thousands of ppm are dangerous and not representative of a TLV.

Step-by-Step Solution:
Compare options against typical occupational hygiene scales (single-digit ppm vs. hundreds/thousands).Select the historically taught TLV level used in many problem sets: 5 ppm.

Verification / Alternative check:
Contemporary standards (e.g., short-term exposure limits) often fall at or below a few ppm. Values in the hundreds or thousands indicate acute hazard conditions, not permissible limits.

Why Other Options Are Wrong:

• 500/1000/2000 ppm: dangerously high; these are emergency/accident levels, not TLVs.
• 0.5 ppm: seen in some modern guidelines but not the classic value targeted by this question style.

Common Pitfalls:
Confusing historic exam values with the latest jurisdiction-specific standards; always check the context and syllabus year.

Final Answer:
5 ppm