Spray Booth Ventilation: How Much Airflow You Need and How to Vent It

Step 1: work out the airflow you need

Required CFM = front opening (sq ft) x face velocity (fpm)

1. Measure the booth's open front: width and height, in inches.
2. Multiply them and divide by 144 to get square feet.
3. Multiply by 150 fpm (OSHA's small-booth design figure for hand-held spray guns with crossdrafts up to 50 fpm), or by 100 fpm for the minimum average in OSHA's 1910.107.

Front opening	Area	CFM at 100 fpm	CFM at 150 fpm
16 x 12 in	1.33 sq ft	133	200
20 x 13 in	1.81 sq ft	181	271
22 x 15 in	2.29 sq ft	229	344
24 x 18 in	3.00 sq ft	300	450

Hobby booth listings usually give airflow in cubic meters a minute. One cubic meter is 35.31 cubic feet, so 5.2 m³/min is about 184 CFM and 9.2 m³/min about 325 CFM. Our spray booth roundup converts each booth's rating.

A worked example

Take a booth rated at 9.2 m³/min (about 325 CFM) with a front opening of 20 by 13 inches. The opening is 260 square inches, or 1.81 square feet. At OSHA's 150 fpm design figure it needs about 271 CFM; at the 100 fpm minimum, about 181 CFM. The fan's free-air rating of 325 CFM leaves some margin for the filter and a short hose, but not much for a long, bent duct. With a 24 by 18 inch opening (3 square feet), the same fan falls short of 150 fpm (450 CFM) even before losses.

Step 2: allow for losses

Fan ratings are free-air figures, measured with nothing in the way. A filter, a long hose, bends and a window insert all cut the real airflow, and the loss grows as filters load with paint. OSHA's rule for workplace booths says filters must be replaced or cleaned whenever the pressure drop across them becomes excessive, and serviced if face velocity falls below its table. In a hobby room that means: buy a booth with clear headroom over your required CFM, keep the hose short, and change filters before they look clogged.

Step 3: vent outside

• Through a window. Run the hose to a window and seal the gap around it with a panel or insert, so exhaust does not blow back in. Master Airbrush's B3000 has a narrow hose end for a window or door gap (per its listing).
• Keep it short and straight. Every foot and every bend costs airflow.
• Point it away from intakes. Keep the outlet away from other windows, doors and air intakes.
• Recirculating booths (filter only, no hose) catch particles but return vapor and odor to the room. Treat them as particle catchers, not ventilation.

Step 4: let air in

A fan cannot pull air out of a closed room. OSHA's rule calls for clean make-up air equal to the volume exhausted in workplace booths. At home, open another window or door on the far side of the room. Keep crossdrafts down at the booth itself: OSHA's table drops its allowance as crossdrafts rise and says crossdrafts over 100 fpm should not be permitted, because a breeze across the opening pushes overspray back out.

Step 5: solvent paints need more care

OSHA's spray finishing rule (1910.107) is written for flammable and combustible finishing materials. It bars open flames and spark-producing equipment in a spraying area or within 20 feet of it, bars portable electric lamps in the spray area during spraying, and requires workplace exhaust to keep solvent vapor at or below 25% of its lower explosive limit (1910.94). Hobby booths are sold for water-based paint; if you spray lacquers, enamels or urethanes, read the paint's safety data sheet and the booth maker's manual first, and keep heaters, pilot lights and tools that spark away from the area. The same rule keeps electric motors that drive exhaust fans out of booths and ducts, and has freshly sprayed work dried only in ventilated spaces, so leave the booth fan running while the paint dries.

Step 6: filters and housekeeping

• Replace booth filters when they load up; keep spares.
• OSHA's rule has used filter pads moved to a safe, detached place, or kept in a water-filled metal container until disposal, because paint-soaked filters can be a fire risk.
• Put rags soaked with paint or thinner in a metal can; OSHA's rule calls for approved metal waste cans for them in workplaces, emptied at least once a day.
• Wipe overspray off the booth walls regularly; OSHA calls for spraying areas to be kept free of combustible residue.

Ventilation is not a respirator

Even a good booth leaves some mist around the opening where you work. Createx recommends a NIOSH or MSHA approved respirator any time you paint. NIOSH notes that filtering facepiece respirators do not protect against gases or vapors, so solvent paints need an organic vapor cartridge. See respirators for airbrushing, and for large jobs, inflatable paint booths.