CONTROL OF SILICA DUST IN CONSTRUCTION 

Large Drivable Milling Machines 

(Half Lane and Larger)

Using large drivable milling machines (half lane or more) on asphalt pavement, 

concrete, and other silica-containing materials can generate respirable crystalline silica

dust. When inhaled, the small particles of silica can irreversibly damage the lungs. This 

fact sheet describes dust controls that can be used to minimize the amount of airborne 

dust when using large drivable milling machines as listed in Table 1 of the Respirable 

Crystalline Silica Standard for Construction, 29 CFR 1926.1153.

Engineering Control Method: Exhaust ventilation on drum enclosure with water spray OR

Water spray with surfactant (for cuts less than four inches)

Wet methods reduce the amount of silica dust that 

becomes airborne when using milling machines, 

because they control exposure at the source. 

OSHA has determined that it is necessary to 

supplement water sprays with a dust suppressant 

additive or with exhaust ventilation. Water 

amended with a foam additive or surfactant 

performs better for dust suppression (surfactants 

are essentially equivalent to dish soap). The soap 

breaks the surface tension and softens the water, 

which improves silica dust capture.

Large drivable milling machines can be equipped 

with a combination of water sprays, exhaust 

ventilation and surfactants to effectively control 

silica dust. The exact combination varies with 

milling depth and substrate material. For cuts of:

• Four inches or less in depth on any substrate, 

Table 1 includes two options. Employers may 

use a machine equipped with exhaust ventilation 

on the drum enclosure and supplemental water 

sprays designed to suppress dust; OR they may 

use a machine equipped with supplemental 

water spray designed to suppress dust. In the 

second option the water must be combined with 

a surfactant.

• More than four inches on asphalt only, Table 1 

requires the use of exhaust ventilation on the 

drum enclosure and supplemental water sprays 

designed to suppress dust.

• More than four inches in depth on substrates 

other than asphalt are not included on Table 1. 

The control techniques described above can be 

used to reduce dust exposures, however, the 

employer must conduct an exposure assessment 

and may need to take additional actions.

Photo courtesy of NIOSH

Cutting drum with enclosure on large milling machine.

Photo courtesy U.S. Air Force, Beth Holliker

Milling machine milling asphalt road and loading 

debris into haul truck. 

FactSheet
Wet Methods

Large milling machines currently come equipped 

with water spray systems for dust suppression. 

The machine must be operated and maintained to 

minimize dust emissions. Make sure to:

• Rinse or replace water filters according to the 

manufacturers’ instructions to ensure they are 

clean and not clogged.

• Adjust the location and orientation of spray 

nozzles to direct water to the front of the 

cutter drum, primary (collection) conveyor, 

secondary (loading) conveyor, discharge pipe, 

and transfer points.

• Check that nozzles are not clogged and spray 

patterns effectively suppress dust.

• Conduct routine inspections to be sure that the 

system components are working properly.

Exhaust Ventilation

The drum housing and conveyors on the milling 

machine enclose the cutter drum and conveyor 

belts. A well-enclosed drum housing and conveyor 

system can reduce workers’ dust exposure. Typical 

ventilation controls designed to reduce dust 

emissions from a piece of equipment consist of a 

hood, fan, ductwork, and dust collector. 

Employers are responsible for keeping equipment 

in good working condition to minimize dust. 

Employers must also ensure that workers are 

properly trained on operating the equipment and 

reducing exposures through good work practices. 

Focus on the following:

• Ensure that the ventilation control has enough 

velocity to prevent dust from settling and 

plugging the flow.

• Check flashing placement at transfer points and 

making sure it is in good working condition.

• Check to ensure that there are no gaps or leaks 

around conveyor enclosures and ductwork.

• Replace worn cutting teeth.

• Conduct routine inspections to be sure that the 

system components are working properly.

Respiratory Protection

When properly used, wet methods and exhaust 

ventilation can effectively control exposure 

to silica dust. Therefore, Table 1 in the silica 

standard for construction does not require use 

of respiratory protection when operating large 

drivable milling machines equipped with the dust 

controls described in this fact sheet.

Additional Information

For more information, visit www.osha.gov/silica

and see the OSHA Fact Sheet on the Crystalline 

Silica Rule for Construction, and the Small Entity 

Compliance Guide for the Respirable Crystalline 

Silica Standard for Construction.

OSHA can provide compliance assistance 

through a variety of programs, including 

technical assistance about effective safety and 

health programs, workplace consultations, 

and training and education. OSHA’s On-Site 

Consultation Program offers free, confidential 

occupational safety and health services 

to small and medium-sized businesses in 

all states and several territories across the 

country, with priority given to high-hazard 

worksites. On-Site consultation services are 

separate from enforcement and do not result 

in penalties or citations. To locate the OSHA 

On-Site Consultation Program nearest you, 

visit www.osha.gov/consultation.

How to Contact OSHA

Under the Occupational Safety and Health Act 

of 1970, employers are responsible for providing 

safe and healthful workplaces for their employees. 

OSHA’s role is to ensure these conditions for 

America’s working men and women by setting 

and enforcing standards, and providing training, 

education and assistance. For more information, 

visit www.osha.gov or call OSHA at 1-800-321-

OSHA (6742), TTY 1-877-889-5627.

DSG FS-3934 12/2017

This is one in a series of informational fact sheets highlighting OSHA programs, policies or 

standards. It does not impose any new compliance requirements. For a comprehensive list of 

compliance requirements of OSHA standards or regulations, refer to Title 29 of the Code of Federal 

Regulations. This information will be made available to sensory-impaired individuals upon request. 

The voice phone is (202) 693-1999; teletypewriter (TTY) number: (877) 889-5627.