Ventilation and local exhaust ventilation

General ventilation by dilution

General ventilation consists of supplying outside air and extracting air from the premises to dilute pollutants and maintain acceptable temperature, humidity, and air quality conditions. It can be natural (openings, windows, skylights, chimney effect) or forced (supply or exhaust fans), and its effectiveness depends on the airflow rate, air distribution, the location of inlets and outlets relative to heat sources and workstations, and the absence of short circuits and dead zones. Royal Decree 486/1997 requires a minimum air renewal rate of 30 cubic meters of clean air per hour per worker in sedentary jobs in non-hot and non-polluted environments, and 50 cubic meters in other cases. The Regulation on Thermal Installations in Buildings establishes ventilation rates according to the required indoor air quality.

Dilution is only appropriate when the contaminant is of low toxicity, is emitted uniformly, in small quantities, and far from the breathing zone, and when there are no identifiable sources; it is not acceptable as the sole measure against highly toxic, carcinogenic, or sensitizing agents, or those emitted in large quantities, nor against dusts and fumes that settle in the environment. INSST Technical Standard 741 establishes the design criteria for general dilution ventilation, and NTP 742 addresses general building ventilation.

In addition to controlling exposure, general ventilation contributes to thermal comfort and indoor air quality, and must compensate for the air extracted by localized extraction systems to avoid depressions that reduce their effectiveness, cause annoying drafts or carry away combustion gases.

Localized extraction: elements and design

• Collection element. Surrounding hood, booth, display case, articulated arm, slit, or ceiling hood; the more it encloses the source and the closer it is to it, the lower the flow rate required to capture the contaminant. The capture velocity at the point of emission must exceed the dispersion velocity of the contaminant and the ambient air currents.
• Ducts. Designed with transport velocities sufficient to avoid deposits (especially with dust), with short routes, few curves and cleaning access points, and with materials compatible with the contaminant.
• Purification. Bag or cartridge filters, cyclones, scrubbers, activated carbon filters or high-efficiency filters depending on the contaminant, with periodic maintenance and replacement and control of the recirculation of the purified air.
• Fan. Selected for the flow rate and pressure drop of the system, located after the purification when the contaminant is aggressive, and with protection against explosive atmospheres where appropriate.
• Discharge. Outlet to the outside at a sufficient height and distance from air intakes, windows and passageways to prevent the re-entry of the contaminant.
• Compensation air. Supply of air equivalent to that extracted to maintain pressure balance and the efficiency of the intakes.
• Integration with the process. Intakes that do not hinder the work, interlocks that prevent operation without extraction, and consideration of the ergonomics of the workstation so that the system is actually used.
• Reference design. Criteria of the American Conference of Governmental Industrial Hygienists (Industrial Ventilation) and of the INSST NTP 672 and 1200 on localized extraction and laboratory fume hoods.

Checking, maintaining and verifying effectiveness

• Reception. Initial measurement of flow rates, capture velocities and static pressures to verify that the system meets the design and to establish reference values.
• Routine checks. Periodic measurement of static pressure at reference points (NTP 615) and flow rate (NTP 668) to detect obstructions, leaks, saturated filters or fan wear; visual checks with smoke to confirm capture.
• Maintenance. Cleaning of pipes and intakes, replacement of filters, inspection of belts, bearings and motors, and recording of interventions.
• Hygiene verification. Measurements of worker exposure to confirm that the system maintains concentrations below limit values ​​and to detect deterioration.
• Modifications. Any change in the process, in the intakes or in the distribution of the premises requires reviewing the design and verifying the effectiveness again.
• Training. Workers must know how the system works, the correct position relative to the intake, the fault signals, and the prohibition of working without extraction when required.

Organizational application: how to manage ventilation and local exhaust

1. Identify in the risk assessment the sources of pollutant emissions and the areas with air renewal needs, and decide for each one between localized extraction, general ventilation or both.
2. Design or commission the design of the systems using recognized technical criteria, considering the type of contaminant, the process, the make-up air and the possible presence of explosive atmospheres.
3. Check flow rates, capture velocities and pressures at the receiving point, and record the reference values.
4. Establish the routine checks and maintenance program, including responsible parties, frequency, and records.
5. Periodically verify effectiveness by measuring worker exposure and adjust the system when the results require it.
6. Train workers in the correct use of the intakes and in the detection of faults, and establish interlocks or procedures that prevent working without extraction.
7. Review the design in response to changes in process, products, or distribution.

Preventive management software allows you to inventory ventilation and extraction systems with their reference values, plan checks and maintenance, record pressure, flow and exposure measurements and alert of deviations from the design.

Limits and common mistakes

1. Use general ventilation when dealing with toxic or carcinogenic contaminants that require localized extraction or closed systems.
2. Install collection systems far from the source or poorly oriented, which do not capture the contaminant before it reaches the breathing zone.
3. Do not provide compensating air, with depressions that reduce efficiency and generate currents.
4. Neglecting maintenance: saturated filters, clogged ducts, loose belts, and flow rates far below design levels.
5. Do not verify effectiveness with measurements after installation or periodically.
6. Recirculating air without proper purification or discharging extracted air near air intakes and windows.

The technical design and verification criteria are detailed in the INSST NTPs and in the reference guides; this sheet is for informational purposes.

Regulatory and reference framework

• Royal Decree 486/1997, of April 14. Workplaces ; Annex III on environmental conditions and minimum air renewal.
• Royal Decree 374/2001, of April 6. Chemical agents; hierarchy of measures with priority given to technical control measures such as localized extraction.
• Royal Decree 665/1997, of May 12. Carcinogenic agents; closed systems and local exhaust ventilation as required measures.
• NTP 741: General ventilation by dilution (INSST) . Design criteria and limitations of general ventilation.
• NTP 672: Localized extraction in the laboratory (INSST) . Elements and design of localized extraction systems.
• NTP 615: Measurement of static pressure for the routine verification of local exhaust ventilation systems (INSST) . Routine verification of the effectiveness of the systems.

NTP 668 of the INSST deals with the measurement of flow in localized extraction systems, NTP 742 with the general ventilation of buildings and NTP 1200 with gas cabinets; the Regulation of Thermal Installations in Buildings sets the ventilation flows according to the required indoor air quality.