APPENDIX D

Other Hazardous Gases

Carbon Dioxide

Carbon dioxide (CO2) is commonly found dissolved in hydrothermal fluids and could be released from pore water when cores are recovered. CO2 is usually considered inert and is heavier than air (1.5 times), concentrating in low areas of quiet air.

Carbon dioxide gas is toxic in higher concentrations but much less so than H2S (see Table AT2). The threshold limit of CO2 is 5000 ppm. Short-term exposure to 50,000 ppm (5%) is reasonable. This gas is colorless and odorless and can be tolerated in relatively high concentration.

Air containing 5% CO2 will cause disorientation in a few minutes. Continued exposure to CO2 after being affected will cause convulsions, coma, and respiratory failure. If a massive flow of CO2 from the well bore should occur (highly unlikely), the principal danger would be from asphyxiation due to lack of air.

Using a BA will provide adequate oxygen until the emergency can be controlled.

Table AT2. Chemical vapor toxicity.
Common
name
Chemical
formula
Specific gravity
(air = 1)
Threshold* limit
(ppm)
Hazardous†
limit
(ppm/hr)
Lethal‡ concentration
(ppm)
Hydrogen cyanide HCN 0.94 10 150 300
Hydrogen sulfide H2S 1.18 10 250 600
Sulfur dioxide SO2 2.21 2 — 1,000
Chlorine Cl2 2.45 1 4 1,000
Carbon monoxide CO 0.97 50 400 1,000
Carbon dioxide CO2 1.52 5,000 5% 100,000
Methane CH4 0.55 90,000 >5% in air —

Notes: * = concentration at which it is believed that all workers may repeatedly be exposed, 8 hr/day, every day, without adverse effect. † = concentration that may cause death. ‡ = concentration that will cause death with short-term exposure.

Hydrocarbon Gas

Hydrocarbon compounds are known to be present in sediments and potentially in hydrothermal fluids. These hydrocarbons are primarily in the form of methane gas, but some heavier gases and even liquids are possible.

The toxicity of methane is very low (see Table AT2), but it is often a "carrier" of H2S. Because hydrocarbon gas is lighter than air, it does not tend to settle and is easily dispersed. The major threat from natural gas is its extreme flammability and explosivity. Hydrocarbon liquids may be carcinogenic, so common sense should be used in handling them if they are encountered.

The blowout control measures specified in this document can be use to control hydrocarbon gas as well as H2S.

Radon

The barite precipitated in seafloor rocks and sediments by hydrothermal activity has been found to contain measurable amounts of radioactive radium-226, which is chemically similar to barium. Small amounts of radon may also be present in the pore water.

The radium emits very small amounts of radon-222 gas through radioactive decay, which also is radioactive. The radiation is in the form of alpha particles, which have extremely limited ability to penetrate tissue. Thus, the principal danger comes from inhaling or ingesting the radioactive material. Furthermore, the levels of radioactivity are extremely low—on the nannocurie scale.

ODP has investigated the potential effect of that radiation on health and safety on board the ship. Nevertheless, cores will be monitored for radioactivity if radioactive material is present.

The following precautions are to be taken if radon is encountered:

Sulfur Dioxide

Sulfur dioxide is produced during the burning of H2S. Sulfur dioxide is colorless, transparent, nonflammable, heavier than air, but will be picked up by a breeze and carried downwind at elevated temperatures, and extremely irritating to the eyes and mucous membranes of the upper respiratory tract.

Table AT3 indicates the toxic nature of the gas.

Table AT3. Effects of sulfur dioxide.
Concentration of SO2
Effect
(%)
(ppm)
0.0002 2 Safe for 8 hr
0.005 3–5 Pungent odor; normally a person can detect SO2 in this range
0.0012 12 Throat irritation, coughing, chest constriction, eyes tear and burn
0.015 150 So irritating that it can only be endured for a few minutes
0.05 500 Causes a sense of suffocation, even with first breath.

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