IV. Processes in the Workplace That May Be Sources of Air Pollutants
The main production processes are the most easily noticed sources of pollution: arc fumes from welding stations, solvent vapors from paint booths, grinding dust from grinding areas, silica-containing dust from foundry shakeout points, and sawing and sanding dust from woodworking shops. These processes usually have fixed workstations, are easy to identify, and are often the focus of LEV design.
Auxiliary processes should not be overlooked. Temporary welding during equipment maintenance, grinding of fixtures and molds, waste material cleaning, and sweeping operations are intermittent operations that are often excluded from regular control. However, workers operating these processes – maintenance workers, assistants, and cleaners – are also exposed to high concentrations of pollutants, sometimes even more severely than operators at fixed workstations, as they do not have targeted protective measures.
Material transfer points are another easily overlooked source of pollution. When powder falls from a height into a hopper, at transfer points of conveyor belts, packaging openings, and vibrating screens, pollution at these locations is often not noticed because they do not belong to any “major equipment”, but over the long term, their pollution impact can be very significant.
Storage areas for raw materials and waste should also be included in the inspection scope. Ruptures in powder bags, uncovered containers, and waste piles – these static sources may have little impact when there is no wind, but the airflow generated by forklifts passing by or doors opening can cause them to release pollutants.
The correct way to identify pollution sources is to stand in the workshop during the busiest production period and observe the path of air flow and dust movement by using a smoke pipe or a flashlight at an oblique angle. Follow the air flow to see where the pollutants are generated, where they go and where they stay. Such on-site observation is far more effective than looking at the drawings.
V. The Needs of Operators Working Near These Pollution Sources
The LEV system is designed for the operators, not for inspectors to look at or to meet audit requirements. Human factors must be taken into account during the design process.
The first need of operators is: not to be harmed. This means that the LEV must be truly effective, and the concentration of pollutants must be reduced to a level below the safety threshold. This is the bottom line and there is no room for negotiation. If workers can still smell pungent odors, see smoke and dust floating in front of their eyes, or feel throat irritation while standing at their workstations, then no matter how “up to standard” the system parameters are, in the eyes of the workers, the system is ineffective. Employers should verify the effectiveness of the system through two dimensions: objective data (air sampling results) and subjective feelings (worker feedback).
The second need of operators is: not to be in the way. If the exhaust hood blocks the operator’s view, making it difficult for them to see the workpiece and weld points, they cannot work normally. If it occupies the operating space, making it difficult for the workpiece to enter and exit, the work efficiency will decrease. No matter how good the design is, if it is not user-friendly, it will eventually be a decoration.
The third need of operators is: not to be uncomfortable. If the airflow generated by the exhaust hood directly blows on the operator, especially in winter, it will cause local coldness and discomfort. If the noise of a large fan exceeds 85 decibels, long-term exposure can lead to hearing damage and fatigue. What workers need is silent and efficient ventilation, not strong winds that oppose them. During the design process, attention should be paid to: the airflow direction of the exhaust hood should avoid facing the operator’s face and torso as much as possible. If it must pass near the operator, consider setting up deflector plates or wind curtains. When selecting fans, low-noise models should be given priority. If necessary, silencers or soundproof enclosures should be installed.
When employers put forward demands and design plans, they should involve the operators. Ignoring the experience from the front line, the system designed is likely not to be used correctly. This is not a technical issue, but a human one.
VI. Can LEV Alone Provide Adequate Control Effect?
LEV is an engineering control measure and the second level in the hierarchy of industrial hygiene controls (only after elimination and substitution). However, it may not be able to solve the problem alone. Employers need to honestly assess whether this system, if operating well, can alone reduce exposure levels below the occupational exposure limit.
In some cases, the LEV itself is the solution. For a welding station where the weld point is fixed and the workpiece is not large, the upper suction hood can be placed 15-25 cm directly above the weld point, and the air volume can be designed based on a control wind speed of 0.5-1.0 m/s, without strong lateral air flow interference – then the LEV can independently control the smoke and dust to a safe level. For a woodworking table saw with a lower suction port under the saw blade and an upper suction hood above, both suction routes operating simultaneously can capture most of the dust generated during sawing. For a chemical laboratory fume hood with an appropriate window opening and a stable face velocity of around 0.5 m/s, the organic vapors produced by operations inside the hood will hardly escape into the room.
However, in most cases, LEV needs to be used in conjunction with other measures. For instance, in large spray booths, LEV should be combined with a reasonable make-up air system. The exhaust fan draws a large amount of air from the spray booth. If no make-up air system is set up (or only natural air enters through the gaps in doors and windows), a high negative pressure will be formed in the room. The negative pressure itself is not a problem. The issue lies in that the air entering through the gaps in doors and windows will have a high velocity, creating local jets. These jets will disrupt the carefully designed airflow in the spray booth, leading to the formation of vortex zones where paint mist and vapor accumulate and cannot be discharged. The correct approach is to install a mechanical make-up air system to supply fresh air that has been filtered and temperature-regulated into the spray booth at a low and uniform speed, maintaining pressure balance.
In the foundry workshop, simple dust removal by shakeout alone may not be sufficient. When the shakeout machine vibrates, a large amount of silica-containing dust is raised. Even with exhaust hoods, some coarse particles of dust will still quickly settle to the ground under the force of gravity. These settled dusts will be raised again during subsequent operations (such as personnel movement, forklift passage, and cleaning), causing secondary dust. Therefore, in addition to LEV (Local Exhaust Ventilation), wet operations (such as sprinkling water or spraying mist to suppress dust) or regular vacuum cleaning to remove settled dust are also required.
For highly toxic substances (such as lead fumes, chromic acid mists, beryllium and its compounds), LEV may need to be supplemented by the use of respiratory protective equipment by operators. This does not mean that LEV cannot effectively control these substances, but rather that given their extremely high toxicity and very low occupational exposure limits, any engineering control system has the potential to fail (such as a fan belt breaking, filter material damage, or a ventilation hood being knocked askew). In such cases, respiratory protective equipment serves as a secondary line of defense. This is a “defense in depth” strategy – not to replace LEV with protective equipment, but to complement it.
There is another situation: LEV may not be the most suitable solution at all. If the pollution sources are extremely scattered – with dozens of sporadic dust-generating points throughout the workshop, and each point generates a small amount of dust. If the workstations are not fixed – workers operate at different positions every day, and installing fixed exhaust hoods cannot cover all the operation positions. If the amount of pollutants generated is very small – only a small amount of volatiles occasionally, and the concentration is far below the limit. Then perhaps process substitution (replacing toxic materials with non-toxic ones), dilution ventilation (increasing the air exchange rate in the workshop), or mobile dust collectors would be more reasonable choices. LEV is not omnipotent, and forcing its use in all situations would lead to wasted investment and poor results.
The core question that employers need to answer is: If this LEV system operates well, can it alone reduce the exposure level below the legal standard? If not, what additional measures are needed? This question should have a clear answer during the design stage, rather than seeking solutions after the system is installed and fails the inspection.