Toxic-gas thresholds (TLV) comparison Among the following, which substance has the lowest threshold limit value (TLV), approximately 0.02 ppm, indicating extreme toxicity at very low concentrations?

Introduction / Context:
Threshold limit values (TLVs) or guideline exposure limits help engineers design ventilation, monitoring, and emergency response plans. Some gases are hazardous at extremely low concentrations due to acute toxicity or strong irritancy.

Given Data / Assumptions:

• Compare typical occupational/environmental guidance values.
• Order-of-magnitude selection rather than an exact standard from a specific year.
• Lowest TLV implies most toxic at trace levels.

Concept / Approach:
Methyl isocyanate (MIC) is infamous for its extreme acute toxicity and very low permissible exposure limits, on the order of a few hundredths of a ppm in many references. Phosgene, bromine, and ozone are also dangerous but typically have higher guideline values (often around 0.1 ppm for ceiling or TWA, depending on the standard). Therefore, MIC is the correct choice for the lowest approximate TLV near 0.02 ppm among the options listed.

Step-by-Step Solution:

Verification / Alternative check:
Historical guidelines and emergency response data sheets consistently flag MIC as requiring the lowest exposure thresholds among common industrial toxics.

Why Other Options Are Wrong:

• Phosgene, Bromine, Ozone: Highly toxic/irritant but with TLVs typically higher than MIC.
• Chlorine dioxide: Very irritating oxidant, yet common limits exceed MIC’s extreme sensitivity level.

Common Pitfalls:
Mixing ceiling, STEL, and TWA values across different agencies; the key here is the relative order—MIC is the lowest among the listed substances.

Final Answer:
MIC (methyl isocyanate)