Industrial hygiene — threshold limit values (TLV) of highly toxic gases What is the typical TLV (time-weighted average) in air for ozone (O3) and for phosgene (COCl2)?

Introduction / Context:
TLV values guide safe workplace exposures to toxic gases. Ozone and phosgene are both highly hazardous even at very low concentrations, so engineers must recognize the correct order of magnitude when designing monitoring and ventilation systems.

Given Data / Assumptions:

• Common TLV figures used in safety training for industrial environments.
• Values are order-of-magnitude indicators; exact limits may vary by standard-setting body.
• Both gases are toxic/irritant at low ppm levels.

Concept / Approach:
Ozone is a powerful oxidant with TLVs in the fraction of a ppm. Phosgene is a severe pulmonary irritant (choking gas) historically used as a chemical warfare agent; its occupational limits are likewise very low. Among the options, 0.1 ppm correctly reflects the low permissible exposure levels for both gases.

Step-by-Step Solution:
Recall that both O3 and COCl2 have limits far below 1 ppm due to acute toxicity.Compare with options: 25, 100, and 1000 ppm are orders of magnitude too high for these gases.Select 0.1 ppm as the appropriate TLV order.

Verification / Alternative check:
Safety datasheets and industrial hygiene references list TLV/PEL values in tenths of ppm or less for these agents.

Why Other Options Are Wrong:

• 25, 100, 1000 ppm: These would represent dangerously high exposures for O3 or phosgene and are not acceptable workplace limits.

Common Pitfalls:

• Confusing alarm setpoints (which may be even lower) with TLVs.

Final Answer:
0.1 ppm