For an occupational health and safety professional in the Mexican construction sector, NOM-033-STPS-2015 is not simply another standard in a safety catalog. It is one of the most technically, operationally, and documentarily demanding regulatory frameworks within México's occupational safety system.** Technical management of confined space work under NOM-033-STPS-2015**
1. Identification and classification of confined spaces
The standard applies to** all workplaces in the national territory** where activities are carried out inside confined spaces, regardless of company size or project type. In the construction sector, the most common examples include:
- Foundation shafts
- Water tanks and cisterns
- Pump sumps
- Septic tanks
- Drainage collectors
- Elevator pits
- Underground tanks or deposits
For an area to be considered a** confined space,** three conditions must be met simultaneously:
- It must have dimensions large enough for a person to enter and perform work.
- It must have limited or restricted means of entry and exit.
- It must not be designed for the continuous occupation of workers.
When the possibility of hazardous atmospheres also exists, the space is classified as a Type II Confined Space, which requires the application of stricter controls. For the occupational health and safety professional, the operational rule is clear: when in doubt, classify the space as Type II, since the standard penalizes underestimation of risk, not excessive caution.
2. Key obligations of the OHS professional
Although the standard establishes obligations primarily for the employer, in practice the **OHS professional is the one who designs, implements, and documents the control system.
**Among the most relevant responsibilities are: ** Identification and registration of confined spaces
**The professional must maintain an up-to-date inventory of all confined spaces on the site, classified by type and with access points marked with visible warnings. ** Task-specific risk analysis ** Each job must have a specific risk analysis, considering:
- Atmospheric hazards
- Mechanical hazards
- Electrical hazards
- Engulfment hazards
- Biological hazards
- Ergonomic hazards
Generic or recycled analyses do not comply with the standard's criteria.
**Work permit system
**The work permit is the central instrument of operational control. In Type II confined spaces, **no work may be performed without a written permit. ** The OHS professional must:
- Design the permit format
- Train supervisors to issue permits correctly
- Maintain permit archives as documentary evidence
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3. Atmospheric monitoring: the most important preventive control
Gas monitoring is the most critical technical element in confined space safety.
Hazardous atmospheres can be odorless, invisible, and undetectable by human senses, which makes multi-gas detectors essential tools.
The basic parameters that must be measured include:
- Oxygen (O₂)
- Flammable gases (LEL)
- Hydrogen sulfide (H₂S)
- Carbon monoxide (CO)
- Carbon dioxide (CO₂)
The recommended protocol includes:
- Measure before any entry.
- Take readings at three levels (high, middle, and low).
- Wait for readings to stabilize.
- Record numerical values on the work permit.
- Repeat measurements if conditions change or if the work extends over time.
A key point for audits is that** detectors must have a current calibration certificate.** Without a calibration certificate, records lose validity during an inspection.
4. Emergency and rescue plan
NOM-033 requires that each confined space have a specific rescue plan. This plan must include:
- Non-entry rescue procedures
- Entry rescue procedures
- Rescue equipment available on site
- Communication chain with emergency services
- Simulation drill records
A fundamental rule that the OHS professional must convey in all training sessions is:
**The attendant must never enter the space to rescue a worker without first activating the rescue system and without the appropriate equipment. ** The majority of fatal accidents in confined spaces involve improvised rescue attempts.
5. Documentary evidence for STPS audits
Regulatory compliance is not demonstrated through declarations, but through **verifiable documentation.
**The safety file must include:
- Up-to-date confined space inventory
- Documented risk analyses
- Formal confined space work procedure
- Specific rescue plans
- Archived work permits
- DC-3 training certificates for personnel
- Detector calibration certificates
- Simulation drill records
- Training evidence
A good practice on site is to maintain a dedicated folder for confined spaces, organized chronologically.
6. Most frequent mistakes on site
STPS inspections commonly identify recurring mistakes:
- Classifying all spaces as Type I to simplify management.
- Reusing work permits throughout the entire workday.
- Measuring gases only at the entry point of the space.
- Designating attendants who are also performing other tasks.
- Generic training with no differentiation by role.
- PPE selection based on availability rather than specific hazards.
- Rescue plans that have never been practiced.
- Out-of-date confined space inventories.
The OHS professional must act as a preventive system manager, correcting these failures before they become incidents or sanctions.
Download the full Confined Spaces guide.
Conclusion
NOM-033-STPS-2015 provides a clear framework for managing safety in confined spaces. However, its effectiveness depends entirely on its practical application in the field.
In the construction sector, one of the greatest risks is the normalization of unsafe practices under the logic of "we've always done it this way." This operational culture is incompatible with high-risk work such as that performed in confined spaces.
The OHS professional plays an essential role in breaking that inertia. Their function is not limited to preparing documents, but to designing real control systems, providing adequate training to personnel, verifying working conditions, and ensuring that every entry into a confined space is backed by effective technical and administrative controls. When the system works correctly, the result is simple but fundamental: every worker who enters a confined space exits it safely.
