What is Regulatory VOC Content?
In environmental compliance, there is a distinct difference between 'Actual VOC' and 'Regulatory VOC' (also known as Coating VOC or VOC Less Water).
Actual VOC is simply the weight of VOCs per volume of the total coating. Regulatory VOC, however, subtracts the volume of water and the volume of exempt solvents from the denominator. This prevents manufacturers from meeting emission limits merely by diluting a high-VOC paint with water. Regulatory VOC is the metric used to determine if a coating complies with local emission limits.
Understanding EPA Method 24
EPA Method 24 is the standard laboratory procedure used to determine the volatile matter content, water content, density, and volume of solids in a surface coating.
A lab will bake a coating sample to determine the total weight of volatiles. It then uses gas chromatography to measure the specific water weight and exempt solvent weight. Exempt solvents are specific compounds, like acetone, that the EPA has determined do not significantly contribute to ozone formation.
How to Calculate VOC Less Water and Exempt Solvents
To calculate the Regulatory VOC content in grams per liter (g/L), you need the following data from the manufacturer's formulation or lab test: Weight of total volatiles (Wv), Weight of water (Ww), Weight of exempt solvents (Wex), Volume of the coating (Vm), Volume of water (Vw), and Volume of exempt solvents (Vex).
The formula is: VOC (g/L) = (Wv - Ww - Wex) / (Vm - Vw - Vex).
The numerator represents the mass of the actual VOCs (total volatiles minus water and non-ozone-depleting exempts). The denominator represents the volume of the solid coating plus the VOCs, removing the volume space taken up by water and exempts.
Calculation Example: Regulatory VOC
Let's evaluate a 1-liter sample of water-based paint. The lab finds: Total volatile weight (Wv) = 400g. Water weight (Ww) = 300g. No exempt solvents are present (Wex = 0g). The volume of the water (Vw) is 0.3 liters.
The mass of the actual VOC is 400g - 300g = 100g.
The volume of the coating less water is 1.0L - 0.3L = 0.7L.
Regulatory VOC = 100g / 0.7L = 142.8 g/L.
Notice how the Regulatory VOC (142.8 g/L) is higher than the Actual VOC (which would simply be 100g / 1.0L = 100 g/L). This ensures environmental limits are applied to the active coating components.
Frequently asked questions
What is an exempt solvent?
Exempt solvents are chemical compounds, such as acetone or methyl acetate, that have been scientifically proven to have negligible photochemical reactivity, meaning they do not contribute meaningfully to smog formation.
How do I convert VOC from g/L to lb/gal?
To convert grams per liter (g/L) to pounds per gallon (lb/gal), you divide the g/L value by a conversion factor of 119.83.
Why do we subtract water volume in the calculation?
Subtracting water volume calculates the VOC concentration within the 'solids plus VOC' portion of the paint. This prevents formulators from simply watering down a non-compliant paint to pass the threshold.
Can I use manufacturer SDS to find the VOC?
Safety Data Sheets (SDS) often list VOC content, but you must verify whether the listed value is the 'Actual' or 'Regulatory (Less Water)' VOC to ensure environmental compliance.