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What are the hazards most commonly found in manholes?

The most common hazards which can sometimes be found in manholes are atmospheric in nature, including vapors or gases such as:

• Gasoline (i.e. from leaking underground gasoline tanks)
• Natural gas (i.e. from ruptured gas mains)
• Liquid petroleum gas or propane (i.e. from storage cylinders)
• Nitrogen (i.e. from tanks used to temporarily pressurize telco cables)

Gases from the decaying of naturally occurring matter such as:

• Methane (very common at or near land-fills)
• Carbon dioxide
• Hydrogen sulfide

Gases created as a by-product of combustion from vehicles or equipment such as:

• Carbon monoxide (i.e. from vehicle or generator exhaust)

Gases used on the job during manhole work operations such as:

• Acetylene
• Propane

Oxygen Levels

• Oxygen depletion
• Oxygen enrichment

What are some of the affects of these hazards?

Carbon Monoxide (CO)

PPM   Resulting Condition
35   OSHA Permissible Exposure Level (PEL)
200   Mild headaches (3-5 hrs)
400   More severe headache and nausea (1-2 hrs)
800   Severe headache, dizzy, nausea (3/4 hr). Collapse & possibly death (2 hrs)
1600   Severe headache, dizzy, nausea (20 min). Collapse % possible death (2 hrs)
3200   Severe headache, dizzy, nausea (5-10 min). Danger of death within 30 mins.

Hydrogen Sulfide (H2S)

PPM   Resulting Condition
0.13   Minimal perceptible odor like rotten eggs
4.60   Strong odor of rotten eggs
10.0   Mild eye irritation
27.0   Strong unpleasant odor, but not intolerable
100   Coughing, severe eye irritation, loss of sense of smell after 2-5 min
200 – 300   Marked eye inflammation & respiratory tract irritation after 1 hr
500 – 700   Unconsciousness & possible death within 1 hr

Gasoline, natural gas, LPG, acetylene, propane
All are extremely combustible. Propane for example, can also displace oxygen

An asphyxiant which displaces oxygen

Oxygen depletion or “Dead Air”
The normal fresh air we breathe consists primarily of approximately 78.1% Nitrogen and 21.9% Oxygen. The remainder consists of low levels of other gases such as Argon, Carbon Dioxide and Carbon Monoxide

Oxygen depletion may occur due to:

• Poor natural ventilation in a closed manhole
• Natural gas (i.e. from ruptured gas mains)
• Natural occurrences (i.e. excessive rust)

Oxygen depletion can be harmful or fatal as indicated in the following chart:

% Oxygen   Resulting Condition
21.9 %
  Normal fresh air
19.5 %
  Minimum safe level
16 %
  Disorientation, impaired judgment & breathing
14 %
  Faulty judgment, rapid fatigue
8 %
  Mental failure, fainting
6 %
  Difficulty breathing, death within minutes

Oxygen enrichment
23 % or more of oxygen can significantly increase the danger of fire or explosion.

What can I do to protect myself from these hazards?
Personnel working in manholes must be thoroughly trained and equipped to work safely when the atmospheric hazards described above may be present. The OSHA Standards for the telecommunications industry (1910.268) and power companies (1910.269) do vary somewhat in regard to manhole work. Therefore, the specific industry safety practices and equipment used for manhole work may also be somewhat different. This is especially true for manhole atmospheric testing and monitoring procedures. For that reason, we are providing a synopsis of these practices by industry in our companion article “Teck Tips”. Please refer to the Teck

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