CMC Chemical Hazard & Risk Management 2 — Questions and Answers
Question 1: A confined space entry permit lists benzene as a contaminant at 4 ppm. The OSHA PEL-TWA for benzene is 1 ppm. What is the correct action before authorizing entry?
- Enter immediately using air-purifying respirators rated for organic vapors
- Ventilate the space and verify benzene drops below 1 ppm, then re-test (Correct answer)
- Accept the reading since it is below the IDLH of 500 ppm
- Evacuate the vessel and contact the Coast Guard
Correct answer: Ventilate the space and verify benzene drops below 1 ppm, then re-test
Benzene must be reduced below the OSHA PEL of 1 ppm through ventilation and confirmed by re-testing before entry is authorized.
Question 2: Which document provides the hierarchy of health hazard data sources a Marine Chemist should consult when evaluating an unknown shipboard cargo?
- OSHA 29 CFR 1910.1000 Table Z-1
- NFPA 306 Standard for the Control of Gas Hazards on Vessels
- The cargo's Safety Data Sheet (SDS) Section 2 and Section 11 (Correct answer)
- IMO IMDG Code segregation tables
Correct answer: The cargo's Safety Data Sheet (SDS) Section 2 and Section 11
SDS Sections 2 (hazard identification) and 11 (toxicological information) are the primary sources for health hazard data on specific chemicals.
Question 3: During hot work in a tank that previously held crude oil, the atmosphere reads 0% LEL. A Marine Chemist must consider which additional hazard before issuing a Safe Work certificate?
- Excessive nitrogen enrichment from vapor suppression blankets
- Residual liquid pooled under gratings that could vaporize when heated (Correct answer)
- Static electricity from the welder's personal protective equipment
- Carbon dioxide released from scale deposits on tank surfaces
Correct answer: Residual liquid pooled under gratings that could vaporize when heated
Residual liquid hydrocarbons trapped under gratings or in low spots can vaporize rapidly when hot work raises temperatures, creating an explosive atmosphere even when initial readings show 0% LEL.
Question 4: A cargo tank that carried methyl alcohol (methanol) shows 0 ppm on a combustible gas indicator (CGI) but the Marine Chemist still withholds the certificate. What is the most likely reason?
- Methanol is not detectable by most catalytic bead CGIs below its flash point (Correct answer)
- Methanol is non-flammable and poses only a toxicity hazard
- CGIs are calibrated in percent oxygen, not percent LEL
- Methanol requires an explosimeter, not a CGI, for detection
Correct answer: Methanol is not detectable by most catalytic bead CGIs below its flash point
Many catalytic bead combustible gas indicators are poorly sensitive to methanol; a zero reading may be a false negative, so a photoionization or colorimetric method should verify the atmosphere.
Question 5: The Threshold Limit Value-Short Term Exposure Limit (TLV-STEL) for hydrogen sulfide is 5 ppm. This means workers may be exposed to that concentration for a maximum of:
- 8 hours continuously, provided the TWA is not exceeded
- 15 minutes, no more than 4 times per shift with 60-minute intervals between exposures (Correct answer)
- 30 minutes per shift under any conditions
- 60 minutes if supplied-air respirators are worn
Correct answer: 15 minutes, no more than 4 times per shift with 60-minute intervals between exposures
ACGIH STELs permit exposure at the listed concentration for up to 15 minutes, repeated no more than four times per shift, with at least 60 minutes between each short-term exposure period.
Question 6: Which risk matrix element is used to calculate a risk priority number (RPN) in a chemical hazard assessment aboard a vessel?
- Concentration × exposure time × molecular weight
- Severity × probability × detectability (Correct answer)
- Flash point × vapor pressure × LEL percentage
- LD50 × IDLH × permissible exposure limit
Correct answer: Severity × probability × detectability
In FMEA-based risk matrices, the RPN is the product of severity, probability of occurrence, and detectability ratings.
Question 7: A Marine Chemist finds oxygen content at 19.2% in a cargo hold. Which statement best describes this condition?
- The atmosphere is safe; OSHA normal range is 19.5–23.5%
- Entry is prohibited because oxygen-deficient atmospheres begin below 19.5% (Correct answer)
- The space is oxygen-enriched and poses an increased fire risk
- No hazard exists because the difference from 20.9% is within instrument error
Correct answer: Entry is prohibited because oxygen-deficient atmospheres begin below 19.5%
OSHA defines an oxygen-deficient atmosphere as less than 19.5% oxygen by volume, making the space an immediately dangerous-to-life environment requiring supplied-air or SCBA.
CMC Chemical Hazard & Risk Management 2
A confined space entry permit lists benzene as a contaminant at 4 ppm.
The OSHA PEL-TWA for benzene is 1 ppm.
What is the correct action before authorizing entry?