Health and Safety
Executive / Commission
Local authority circulars
To: Directors of Environmental Health/ Chief Environmental Health Officers of London, Metropolitan, District and Unitary Authorities and Chief Executives of County Councils.
For the attention of: Environmental Services / Trading Standards / Fire Authorities / Other
This circular gives advice to local authority enforcement officers
1 This circular includes a DATA SHEET which gives information about:
2 The attached DATA SHEET is intended as a working document for local authority enforcement officers but it may be copied to employers and other persons.
| Health and Safety Executive | HELA Data Sheet |
1 The Control of Substances Hazardous to Health Regulations 1999 (COSHH) with their emphasis on assessment, prevention and control has brought the control of paint spraying in MVR into sharper focus. This document introduces the aspects of paint spraying that are important for risk assessment and control.
2 Advice on Personal Protective Equipment (PPE) including Respiratory Protective Equipment (RPE) necessary during the spraying of flammable and toxic liquids is outside the scope of this document. This Data Sheet does not cover the health hazards of paints and coatings or hazards of spraying toxic or flammable liquids. The HSE publication HSG178 'The Spraying of Flammable Liquids' provides information on the hazards of spraying with flammable liquids and describes preventive and protective measures to reduce the risk of fire and explosion. Serious health hazards are associated with spraying of two-pack paints and lacquers containing isocyanates. Guidance is given in Guidance Note EH16 'Isocyanates: Health hazards and precautionary measures'.
3 Not all paint spray leaving the nozzle of a spray gun lands on the object being sprayed. Different spray systems are more or less efficient at transferring paint onto surfaces but all create what is called 'overspray'. This consists of relatively large and visible paint droplets which land nearby but not on the object being sprayed, and also fine paint particles which mix with and remain suspended in air. An important property of such fine particle paint aerosol clouds/mists is that they are invisible under normal diffuse lighting. The fine particle paint aerosols will fill the volume of the booth or spray space and will remain airborne for prolonged periods. How long will depend on the particle size and the effectiveness of the ventilation. The table at the end of this document shows the characteristics associated with compressed air-spraying and various alternative techniques.
4 In the spray process, liquid is converted into a fine mist of droplets which is propelled towards a surface where an evenly distributed film is formed. The main spray methods are:
5 Practically all motor vehicle refinishing is carried out using compressed air- spraying equipment. This consists of a compressed air operated gun in which liquid is atomised in either internal-mix or external-mix nozzles by compressed air (typically at 3-6 bars). In internal-mix nozzles liquid and compressed air are combined in a chamber inside the nozzle whereas in external-mix nozzles liquid and compressed air are ejected through separate orifices to combine outside the nozzle. External mixing is said to give a better quality finish.
6 Liquid can be fed to the gun under pressure, under gravity or by syphonic action. The latter two methods require small containers attached directly to the guns and are used when only small quantities are to be sprayed. Syphonic guns are not suitable for high viscosity liquids. Where a large quantity of liquid is to be sprayed, it is fed to the gun under pressure through a hose connected either to a pressure-pot or a pump. The syphonic and gravity feed systems require more compressed air than the pressure fed system and as a result produce more spray mist.
7 Compressed air- spraying is cheap, gives a high quality finish, is versatile and the output is adjustable (typically 0.2 - 0.8 litres per minute). However compared with other methods of spraying (not generally used in motor vehicle refinishing, please see Table 1) it suffers from a number of disadvantages: there is an unavoidably high rate of mist formation.
8 Only about 50% of liquid sprayed reaches its target and therefore several coats may be needed making control relatively difficult and expensive. In the 'mistless' method, the spray is contained within a conical sheath of air which reduces overspray and ricochet.
9 In compressed air-spraying the flow of air carries fine droplets towards the surface being coated. As the air strikes the object it is deflected: droplets of less than about 12 microns diameter tend to be of insufficient mass to be deposited and are deflected by the airstream. This phenomenon is known as 'bounce back'. Approximately 20% of droplets produced by compressed air -spraying are smaller than 12 microns in diameter.
In compressed air-spraying, liquids need to be of a relatively low viscosity leading to relatively large amounts of thinners being used: solvents may then evaporate to give high vapour concentrations for longer periods of time.
Minimising risks by changing spray application methods
10 Other methods, such as airless, hot and electrostatic spraying (see Table I) are occasionally used for motor vehicle refinishing. The scope for large -scale changes of method may be limited because of their high initial cost and colour inaccuracies of these methods. Hot spraying is rare but occasionally found in commercial vehicle refinishing. A 'high pressure low volume' variation of compressed air- spraying is marketed by several companies, more often aimed at the commercial vehicle market, which is claimed to result in higher rates of deposition, and less mist, overspray and bounce-back.
11 It is anticipated that the majority of vehicle refinishers will continue to use the compressed air method because of its cheapness and versatility. Particular attention will therefore have to be paid to the factors which influence the control of spraying although all are relevant to all types of spraying.
12 Relevant factors in the control of spraying include:
13 All controls should be maintained within specification. It is essential therefore at the start of any control regime that, for instance, the air flow volumes and velocities of a booth are well specified and a clear maintenance schedule is developed and applied to ensure the controls meet the specification. Suitable records should be kept of ventilation maintenance.
14 Spray booths may be defined as ventilated enclosures providing a positive movement of air in the spraying area to protect the health of those outside the booth to give some protection to the health of the operator and to provide an atmosphere free from the risk of fire or explosion. The basic aim is to draw clean air past the breathing zone of the operator as effectively as possible before exhausting it after suitable filtration.
15 In general the air flow should be uniform over the whole working area and in a direction such that the operator avoids breathing air contaminated at dangerous levels by the liquid being sprayed. Airflow through a booth should be as linear as possible ie in a down-draught booth for example the air should literally move in a downward direction.
16 The booth should be well lit during the spraying process. Sufficient and suitable lighting should be provided to remove the temptation to spray outside the booth. Fixed lights should be positioned so as to avoid deterioration by overspray and to avoid shadows or dazzling. Guidance is given in HSG178 'The Spraying of Flammable Liquids'. Where work on a vehicle is carried out inside a booth, there should be a clearance of at least 1m between the walls of the booth and the vehicle.
17 Commercial spray booths and enclosures are widely available. There are many different designs depending on the type and size of articles to be sprayed. Spray booths can be broadly classified by:
(A) Side-draught booths
This type of booth is perhaps the one most commonly seen in motor vehicle refinishing. The airflow is horizontal, and for this reason it is sometimes termed as a cross-draught spray booth. The greater the degree of enclosure the easier the control of overspray. The intake air is usually filtered through filters in the vehicle entrance doors and passes down the booth to the opposite end where it is extracted. The inlet filters provide two advantages: they remove particulates which might otherwise deposit on the painted surfaces and reduce the effects of cross-draughts and disturbances which are external to the booth. Doors and other gaps should be sealed or the resultant high-speed air streams entering the booth would disrupt the internal air flow and could entrain dust.
Draughts across an open- fronted booth should be avoided and should not in any case exceed 0.5 metres per second. The flow of air into the booth should be as smooth and unobstructed as possible; Large pieces of equipment should be kept out of the booth to minimise any disruption of the air flow (see Figure 9) which may cause contaminated air to reach the breathing zone of the operators.
(B) Down-draught booths
In this type of booth, the air enters the spraying area through roof filters and travels in a vertical direction towards the floor of the booth where it is exhausted. This can be done via either floor pits or a raised floor. Unlike in side-draught booths, the operator need not be downstream of the aerosol. Both the inlet and exhaust may be powered independently and the booth may therefore be run under positive or negative pressure. Most booths of this type are run under positive pressure as this prevents the inward leakage of dusty air. About 10% more air is supplied than is extracted. The excess air leaks out through gaps under doors for example and will carry some overspray/vapour out of the booth. This mode of working is not recommended for highly toxic finishes such as isocyanate-containing paints where vapour mist and droplets should be contained within the booth. Within down-draught booths fluorescent light fittings are often installed behind oblique diffuser panels in the upper corners and running the length of the booth. Although these give good illumination the larger the oblique panel the greater the disturbance to the downward air flow. For example a 0.5 metre wide oblique panel may bring contaminated air back into the breathing zone of the operator (see Figure 3).
[There is a variant of this type of booth where air is extracted through low-level slots in the side walls of the booth. The momentum of the air entering through the roof filters carries it to the floor of the booth where it is deflected towards the side extracts.]
(C) Semi-down-draught booths
This type of booth can be used where it is not practical to extract from an under-floor pit. Air enters the booth through filters in the roof towards one end of the booth and is extracted through the opposite end of the booth, as in the side-draught booth.
(D) Spray-bake booths
In this type of booth, the paint is cured or baked inside the booth after spraying. It is usual for the bake cycle to be automatic and to include a purge and cool-down phase. The baking can be carried out using infra-red heaters and/or a heated make-up air system (typically 1 million BTU [109 J]). The booth is empty of personnel during this process. During this cycle the air flow rate may be reduced; to reduce heat requirements further, some air may be recirculated but a certain amount of fresh air is required to reduce solvent concentrations.
18 The main types of air cleaning systems used in motor vehicle refinishing are:
(A) Water- Wash
(i) Pumped
In a typical system (see Figure 5), a water curtain is formed by spraying water onto a deflector plate towards the rear of the booth. Overspray impacts on the water film and is washed down into a collection tank. Air is forced through the water curtain at the bottom of the plate where additional sprays may aid the removal of water. Baffles are used to remove water droplets from the air before it passes through the fans. The addition of chemicals to the water causes the paint to break down and collect in the water tank from which it can then be removed.
(ii) No Pump
A high static-pressure fan is used to exhaust air from the booth under a plate at the water's surface. The high velocity air causes the water surface to break up into droplets which scrub the overspray from the air. The advantages of this system are that there are no pumps, pipework or nozzles to block up, no moving parts, lower maintenance costs and booths can be more compact. The disadvantage is that air distribution may not be as good as in some of the pumped designs as air is only removed at the rear bottom edge leading to a stagnation region in the top rear edge.
The water wash system in general has the advantages that the air flow remains constant and the particle removal efficiency is high. It will accept most types of sprayed materials and the running costs may be lower than using filters. In addition there is a degree of fire protection given by the water spray. The main disadvantage is that the initial cost, especially that of the pumped version, is higher than that of the dry system.
The efficiency of both types (up to about 99%) depends on the amount of water circulated; the greater the water flow rate, the greater the particle removal efficiency. The deflector plates should be designed so that large volumes of air pass through small gaps and make sharp changes in direction. This means that the resistance will be high. Within the limits of operating costs, the higher the resistance, the higher the removal efficiency.
(B) Baffles
This is the cheapest most basic and most ineffective method of removing particulates. It relies on impingement of the overspray on the front and back surfaces of steel baffles. These are in the form of either a maze through which the contaminated air must pass making sharp changes in direction or an array which does little more than give a uniform distribution of air over the booth area. This type of booth is very commonly found in motor vehicle refinishing but is only suitable where the quantity of liquid sprayed is small and is intermittent. Its initial cost is low and the air flow rate is constant. However the booth can only be used where it is not necessary to remove particulates completely before the air is exhausted and, because of the high particulate load carried by the exhaust air, deposits will accumulate not only on the baffles but also on the fan impeller and in the ducting.
(C) Dry Filters
Dry filters both remove particles from the exhaust air and distribute the flow evenly throughout the booth. They combine low cost with high efficiency particle removal. There are two forms of dry filtration used in motor vehicle refinishing booths: the replaceable filter cells of pleated paper or fibre matting which fit into steel wire backed panels, and a disposable automatic-feed roll. Dry filters become blocked in time resulting in an increased pressure drop across the filter and a reduction in the air flow rate. Means should be provided to indicate when dry filters need replacement.
In the case of the roll type filter, the filter curtain is automatically wound on and changed when a differential pressure gauge detects a particular pressure drop across the medium. With the filter bank the change is made manually preferably on the indication of, or an alarm triggered by, a differential pressure gauge. Unfortunately the change is probably most usually made on a subjective basis. This type of booth is relatively easy to erect and install and the initial capital cost is low. It does however need regular checks of the filter condition with changes being made as required to maintain efficient operation.
19 The purpose of ventilation is to:
20 The basic parameter used to specify the ventilation requirement or a booth is the control velocity. Manufacturers usually specify the ventilation performance of a spray booth by its volume flow rate. It is sometimes expressed as the volume flow rate per unit area of the booth in the flow direction. This has the dimensions of velocity and should be at least as high as the "control velocity". The control velocity is the mean velocity at the face of open fronted booths or at the operator's position in other types, which is sufficient to overcome turbulent air movements generated both by the process and the random room air currents, and which will draw contaminated air away from the operator's breathing zone.
Mechanical ventilation should be kept on at all times during spraying and for a sufficiently long time after spraying has ceased (and during baking if applicable) to allow vapour from the drying/curing process to be exhausted from the booth [purge time].
(A) Side-draught booths
In HSE publication HSG178 on the spraying of flammable liquids 'a minimum average air velocity of 0.7 m/s at the front of open-fronted booths and enclosures is recommended'. Where two-pack paints containing isocyanates are to be sprayed, additional precautions will be required.
For open-fronted booths where vehicles are sprayed measuring points should be regularly spaced at 3 heights with a minimum of 1 point horizontally for every metre in width of the booth.
Where the operator works inside the booth, the plane of measurement should be where he/she works. The mean of the measurements should not be less than 0.5 m/s with a minimum measured value of 0.4 m/s.
(B) Down-draught booths
Air speeds should be measured at 10 points around the vehicle, 3 on each side and 2 at each end, at 0.5 metres from the vehicle and at a height of 0.9 metres. Where a sprayer works inside this type of booth the mean of these 10 values should be greater than or equal to 0.4 m/s with a minimum measured value of 0.3 m/s. For long booths used to spray heavy goods vehicles measurements may be made at heights of 1.5 metres, 0.5 metres from the vehicle, 2 at each end and others at intervals between 1.5 and 2 metres along the sides. The values which should be achieved are a mean of not less than 0.4 m/s and a minimum measured value of 0.3 m/s as before.
(C) Semi-down-draught booths
These are hybrid structures and are probably best tested as side-draught booths.
(D) Spray-bake booths
Suppliers of proprietary equipment should advise on the appropriate flow rates for the proposed purpose.
Note: The draft European Standard prEN 12215 (1995) would, if adopted, apply to new spray booths for the spraying of organic liquid coatings (paints, varnishes etc.) but would exclude combined spray booths i.e. those in which the spraying and drying of organic liquid coating materials are carried out successively. Although this draft has not yet been adopted as a Standard and must not be regarded or used as a British Standard, it is worth noting that values for air velocity given therein are significantly lower than those given in this document. However the values given in the draft European Standard pr EN 12215 (1995) were for an empty booth and hence the velocities would be lower. In addition, the proposed values in the draft European Standard pr EN 12215 (1995) refer to a different set of measuring points and the plane of measurements etc which may explain the proposed lower air velocity values.
Make-up air
21 All exhaust ventilation systems require replacement or make-up air. Spray booths used in motor vehicle refinishing extract large volumes of air which should be replaced through a well designed system so as not to create high speed/velocity draughts in the area housing the booth. This is especially important where open-fronted booths are used. The distribution of the air flow inside a booth is as important as the average air speed. Air should enter the booth in the same direction as the flow through it so as to avoid cross draughts. The flow direction should be from the sprayer 's breathing zone towards the extract point. Where an adequate volume of planned make-up air is not supplied air will infiltrate through gaps under doors, cracks in walls etc. This may cause high velocity draughts which might interfere with the efficient operation of the booths.
22 During the refinishing of motor vehicles the direction, duration and volume of spray is directly under the control of the operator, whose method of work may determine the success of other control measures being applied. Adequate training in the techniques of spray painting should be provided to teach how to spray with the minimum amount of overspray and bounce-back, to obtain the correct balance between air and liquid flow rates, and to ensure that the minimum pressure for good atomisation is always used.
23 Sprayers should take the following precautions:
| METHOD | RELATIVE COST | TYPICAL SYSTEM PRESSURE | OUTPUT LITRES PER MINUTE | RATE OF MIST FORMATION | BOUNCE- BACK | REQUIREMENT FOR THINNERS | OTHER HAZARDS |
| Compressed air | Cheap | 3-6 bar | 0.2-0.8 1/min | High - only about 50% of paint reaches target | High | High | 2 hoses required equipment heavy |
| Airless (liquid is atomised by forcing it through a very small orifice) | Expensive | 50-400 bar | 0.4-5.3 1/min | Low - about 80% of paint reaches target | Low | Low | Injection risks |
| Electrostatic | Expensive | 3-150 bar | 0.2-5.3 1/min | Low-about 90% of paint reaches target | Low | Low | Electric shock |
| Air-assisted/airless (combines compressed air and airless methods) | Expensive | 30-80 bar | 2-3 1/min | Low - about 90% of paint reaches target | Low | Low | Injection risks |
| Warm air (55-60°C) (used with compressed air) | Moderate | 0.15 bar | 0.2-0.8 1/min | Low | Low | Low | Equipment heavy |
| Hot spraying (60-80°C) (used with airless method) | Expensive | 30-80 bar | 0.4-5.3 1/min | Low | Low | Very Low | Equipment heavy and cumbersome |
'Bounce-back' - air from guns may strike object being sprayed and deflect droplets (particularly small ones < 12 microns diameter) away from the surface.
Figure 3 - Flow Inside a Down-Draught Booth
Figures 4, 5 and 6 - Water Wash Booths (Pumped)
Figure 7 - Water-Wash Booth (No Pump)
Figure 9 - Recirculation Caused by an Obstruction in a Booth
Figure 10 - Semi-Down-Draught Booth
