---

Due to poor ventilation and physical structure, the atmosphere in confined spaces may be actually or potentially hazardous. Atmospheric hazards include the following:

• Oxygen deficient or oxygen enriched atmospheres
• Combustible atmospheres
• Toxic atmospheres
• Any other atmosphere that is immediately dangerous to life or health

Employees trained in atmospheric monitoring will test several points in a confined space for the following:

• Oxygen content
• Combustible atmosphere
• Potential toxic contaminants

Oxygen Monitoring

Oxygen enriched atmospheres are more than 23.5% oxygen; oxygen deficient atmospheres are less than 19.5% oxygen. Certain chemical or biological reactions may reduce oxygen over time, but employee operations such as cutting or welding may reduce oxygen content very quickly. Oxygen levels must be tested regularly whenever hotwork is performed within a confined space. The following graph outlines human reaction to various oxygen levels.

Human Reaction to Oxygen Concentrations

Combustible Atmospheres

Combustible atmospheres have enough oxygen and flammable vapor, gas, or dust to ignite and support a fire or explosion if exposed to flames, sparks, or heat. Oxygen-enriched atmospheres and hazardous atmospheres in excess of their lower flammable limits are extremely combustible and dangerous. The following graphic illustrates the relationship between oxygen, heat, and fuel.

Toxic Atmospheres

Toxic atmospheres can cause injury, illness, or death. Safety concerns include inhalation and skin exposure. If the identity of the toxic atmosphere is known, check all appropriate Material Safety Data Sheets (MSDSs) for threshold limit values and recommended personal protective equipment. If the identity of the toxic atmosphere is not known, use maximum PPE (i.e., SCBA).

---