The problem with fines

Spray drift is the most important issue for the aerial application industry today, largely because of the urbanization of rural America and the increased scrutiny by the Environmental Protection Agency (EPA).

Many factors influence drift potential, but small droplets, also called fines, are the most important. What is the problem with small spray droplets? The smaller the spray droplet, the more likely the droplet will drift, because small droplets fall much more slowly than large droplets, giving wind more time to blow off-target.

All spray nozzles produce a range of droplet sizes. Since droplets are spherical or nearly spherical, their size can be determined by measuring the diameter. There is no absolute cut off on what size of droplet is too small, but very fine and fine droplets are more prone to drift than large droplets (see table). For reference, 100 microns is about the diameter of a human hair.

The most common way to measure spray droplets is Volume Median Diameter (VMD). VMD refers to the droplet size in microns where half of the spray volume is composed of droplets smaller and half is bigger. For example, a VMD of 300 means that half of the volume has droplets the same size or smaller than 300 microns and half is the same size or larger. The most common classifications for agricultural sprays according to their VMD are fine, medium, and coarse.

Droplet size classifications based industry standards (BCPC and ASABE).

Size Category
Approximate VMD
Range (microns)
Very Fine

The most common way that spray drift occurs is when wind unexpectedly blows droplets away from the target. Pesticide labels often forbid application when winds are above a maximum speed such as 10 or 12 mph.

Strong wind is not always the culprit. Many pesticide labels forbid application when wind speeds are less than 3 mph. If there is no wind to mix the air, a temperature inversion may occur when ground air is cooler than the air above. In an inversion, small spray droplets may be suspended and unpredictably move long distances, even by a gentle breeze.

Pesticides must be kept on-target. Pesticide drift has definitely been a problem in the past, and there still are many examples of lawsuits each year where litigants claim a small amount of a pesticide has moved off-target and caused undesirable impacts.

Most experts recognize that trace levels of drift are unavoidable. The EPA has stated, “… some minimum level of drift would occur from most or all applications as a result of the uses of pesticides.” Nonetheless, the EPA is expected to announce stricter drift policies and application requirements on new pesticide labels.

To mitigate drift, pesticide labels are evolving to recommendations based on droplet size. For example, the Roundup PowerMAX® label has two sections with 25 lines of text devoted to droplet size, “Importance of Droplet Size” and “Controlling Droplet Size.” Other labels use words like medium or coarse to define spray droplet requirements.

Applicators should select nozzles and spray pressures based on manufacturers’ recommendations. Results will vary when concentrated product mixtures are applied. In an ideal world, applicators would have tables for all the mixtures that they apply. However, they don’t, and so must take other precautions.

Unfortunately, the different ways to measure droplets do not agree, particularly for fine droplets that are likely to drift. Spray droplet size currently can be measured by three methods: Petri Dishes containing an oil or water sensitive paper, light diffraction and optical methods. The first is very tedious and time-consuming and the other two are laboratory-only techniques. Hence, applicators should attend an Operation S.A.F.E. clinic biannually to check the droplet size they are applying, as well as their spray pattern and other important application characteristics.

New technology to measure droplets is coming. BFS (Essex, UK) is introducing a product with claims that its way to count and measure droplets is more accurate than current technologies, weighs just four pounds and is designed for use in both the laboratory and field. If independent researchers validate these claims, this technology could become a great tool to help commercial applicators characterize their spray applications.

Fine droplets are certainly not “fine” when they drift. Having the spray system setup correctly and using best practices that include proper system setup, low drift formulations and drift mitigation adjuvants will help reduce the production of driftable fines. Even more important to reduce the number of driftable fines is an experienced and well-trained applicator applying sound judgment and best application practices.

For more information on adjuvants and aerial application, contact John Garr at 765-395-3441, or visit



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