An enclosed space means a space which has limited openings for entry and exit, inadequate ventilation, and/or not designed for continuous worker occupancy. Within an enclosed space, as a top priority, the level of oxygen must be checked. On one hand, sufficient oxygen is necessary to support breathing. On the other hand, surplus of oxygen may create flammable condition. Apart from oxygen level, flammable gases level and harmful gases level should be tested accordingly.

How safe atmosphere is defined

As per IACS recommendation No 37, a safe atmosphere contains the following:

  • 20.6% to 22% oxygen by volume content meter and

  • combustible gases - less than 5% of lower explosive limit (LEL), toxics within acceptable limits.

If these conditions cannot be met, additional ventilation must be applied to the space and re-testing should be conducted after a suitable interval.

Gas Limits

Oxygen Concentration %

Effects and Symptoms

20

Minimum safe entry level

16-19.5

Poor coordination, fatigue

12-16

Rapid pulse, difficulty in breathing

10-12

Very fast and deep breathing, lips begin to turn blue, headache

8-10

Fainting, unconsciousness, nausea, vorming

6-8

Fatal in 8 minutes, 50% fatal in 6 minutes

<6

Coma in 1 minute, convulsions, respiratory and cardiac arrest, death

 

Substance

LEL *

(% Vol)

UEL**

(%Vol)

Auto Ignition Temp °C

Flash Point

°C

PEL***

(ppm)

Benzene

1.2

7.1

498

-11.1

5

Carbone Monoxide

12.5

75

607

N/A

25

Hydrogen Sulfide

4.3

44

260

N/A

10

Ethanol

3.5

19

365

13

1000

Styrene

1.1

6.1

490

31

50

Isopropyl Alcohol

2

12

300

11.7

400

Butane

1.8

8.4

287

-60

800

*Lower Explosive Limit (LEL)

**Upper Explosive Limit (UEL)

***Permissible Exposure Levels (PEL).

Key considerations when testing the atmosphere

  • Testing should be carried out by competent individuals using calibrated equipment following manufacturer’s guidance for equipment use.

  • Personnel responsible for atmosphere testing should be aware of calibration routines, required measurement time of the instrument in function of measuring distance, etc.

  • Testing needs to be conducted before any person enters a confined space and at regular intervals until the work is completed.

  • As gasses have different concentrations and weight, the atmosphere of the space should be tested at different levels to obtain a representative sample of the atmosphere. However, depending on the configuration of the space, this may be impossible without entering it at some level.

  • The instrument that is used to test the atmosphere in an enclosed space, should be capable of remote sampling and detection for all gases that it is designed for, without interference from the atmosphere or other characteristics of the intervening space.

  • The use of flexible hoses or fixed sampling lines may assist in reaching remote areas within the enclosed space.

  • Duration of sampling will ensure the full length of the sampling line has been flushed with the atmosphere gases of the space to be measured.

Why space ventilation is vital

Any enclosed space should be properly ventilated before entry, by opening as many access points as possible. Natural ventilation may be acceptable in some circumstances, for example where a small space opens up directly to fresh air.

The time necessary to ensure effective ventilation depends upon the size and construction of the space, the capacity and efficiency of the ventilation system, the level of contamination and the density of the vapour to be displaced. Effective ventilation capacity is also dependent upon the size and position of openings to the space. Well placed openings improve the flow of air and will help ensure that all areas within the space are effectively ventilated.

Ventilation of an enclosed space, apart from permanent systems, may be implemented by additional portable ventilation devises. These devices can either provide fresh air or remove harmful atmosphere. A large diameter ventilation hose in good condition can be attached to a fan and lowered into the enclosed space. The air intake of the hose should be placed in an area that will draw in fresh air only.

Enclosed spaces should not be entered until it is confirmed that the atmosphere is safe and then only for a specific authorised purpose.

Related Guidelines

  • MSC.1/Circ.1485 - Early implementation of SOLAS Regulation XI-1/7 on atmosphere testing instrument for enclosed spaces 

  • MSC.1/Circ.1477 - Guidelines to facilitate the selection of portable atmosphere testing instruments for enclosed spaces as required by SOLAS regulation XI-1/7