Asphalt pavement milling machines use rotating cutter drums during the implementation to remove deteriorated road surfaces for recycling.
Respirable Crystalline Silica
Workers may be exposed to respirable crystalline silica that is released during the removal of the road surface. The exposure to respirable crystalline silica for the operator and ground worker was evaluated in two different half-lane and larger asphalt pavement milling machines that had ventilation dust controls and water sprays designed and installed by the manufacturers and it can cause sickness. Manufacturer a completed milling on four highway construction sites in Wisconsin in eleven days; Manufacturer B completed milling on seven highway construction sites in Indiana in ten days. In order to evaluate the dust controls, full-shift personal breathing zone air samples were collected from an operator and a ground worker during routine employee work activities of milling asphalt pavement at eleven different sites. 42 samples of air from the personal breathing zone were collected over the course of 21 days (sampling by an operator and ground worker each day). 
For respirable crystalline silica, all samples fell below the National Institute for Occupational Safety and Health's recommended exposure limit of 50 g/m3. The geometric mean personal breathing zone air sample for the operator of the Manufacturer of a milling machine was 6.2 g/m3 and 6.1 g/m3 for the ground worker. The geometric mean personal breathing zone air sample for the operator on the Manufacturer B milling machine was 4.2 g/m3 and 9.0 g/m3 for the ground worker. Furthermore, the upper 95% confidence limits for the mean exposure for each occupation in both studies were significantly lower than 50 g/m3. The silica content in the bulk asphalt material ranged from 7% to 23% silica for roads milled by Manufacturer A, and from 5% to 12% silica for roads milled by Manufacturer B. The findings show that on highway construction sites, respirable crystalline silica generated by asphalt pavement milling machines can be controlled by combining ventilation controls and water sprays. Agricultural, foundry, hydraulic fracturing, mining, sandblasting, stone and granite work, and construction all have the potential to expose workers to respirable crystalline silica. Excessive crystalline silica exposure has been linked to a variety of construction-related tasks. Some of these jobs include tuckpointing, concrete sawing, grinding, scabbling, jackhammering, roof tile installation, and abrasive blasting. 
It has also been shown that road milling increases exposure to respirable crystalline silica. To ascertain whether the overexposures were caused by a lack of efficient controls or poor machine maintenance, more information about the operating parameters and engineering controls present on the milling machines is not provided in the three road-milling studies. Asphalt pavement recycling makes use of a range of equipment, including cold-planers, heater-planers, cold-millers, and heater-scarifiers. This article is about cold milling. Cold-milling, which employs a toothed, rotating cutter drum, is frequently used to remove surface deterioration from Portland cement concrete and petroleum-asphalt aggregate road surfaces. Road, street, and bridge construction employ approximately 251,000 people in the United States. A number of these workers operate or live near cold-milling equipment. The machines frequently generate dust containing crystalline silica, which can be inhaled. This respirable dust may be carried by air currents to workers breathing areas near the milling machine. This article's testing was organized by the Silica/Asphalt Milling Machine Partnership as part of a coordinated effort by labor, business, and the government to lessen exposure to respirable crystalline silica during the milling of asphalt pavement for highway construction. The National Asphalt Pavement Association (NAPA) is in charge of coordinating this collaboration between heavy construction equipment manufacturers in the United States and other countries that currently export pavement-milling equipment to the American market. 
The Silica/Asphalt Milling Machine Partnership also includes the Laborers International Union of North America, the International Union of Operating Engineers, the Association of Equipment Manufacturers, and a number of paving contractors. The Occupational Safety and Health Administration (OSHA), the Federal Highway Administration, and the Centers for Disease Control and Prevention (CDC) are among the government agencies represented (NIOSH). The main goal of this article is to evaluate engineering controls designed to reduce silica exposure among workers on half-lane and larger cold-milling machines, which is one of the Silica/Asphalt Milling Machine Partnership's goals. Among the engineering controls investigated in this study were the ventilation controls and water-spray systems used to cool the cutting teeth on asphalt pavement milling machines. Previously conducted field studies assessed the water-spray dust suppression controls. Prior to field testing, the capture efficiency of the ventilation dust controls was evaluated in a factory setting using tracer gas. Each manufacturer improved its silica dust controls for the Partnership. This final stage of testing was designed to confirm the efficacy of the final engineering control configuration before it was installed on an entire fleet of milling machines. Silicosis is a crippling and potentially fatal lung condition caused by inhaling respirable crystalline silica. Silica exposure has also been linked to lung cancer, chronic obstructive pulmonary disease, renal disease, and other negative health outcomes. 
Between 1990 and 1999, at least one-third of silicosis decedents worked in mining or construction. The NIOSH recommended exposure limit (REL) for respirable crystalline silica was determined using the time-weighted average (TWA) of a full-shift personal breathing zone (PBZ) sample, which is 50 g/m3. Most employees who work up to a 10-hour workday during a 40-hour workweek can use this REL to reduce their risk of silicosis, lung cancer, and other negative health effects. The construction industry uses impinger sampling to determine the current OSHA permissible exposure limit (PEL) for respirable dust containing crystalline silica. In the construction industry, the PELs for quartz and cristobalite are identical. Since the adoption of PELs, gravimetric sampling has largely replaced impinger sampling. OSHA currently instructs compliance officers to use a conversion factor when converting between gravimetric sampling and the particle count standard when characterizing exposures from construction operations.
On September 12, 2013, OSHA issued a Notice of Proposed Rulemaking (NPRM) for occupational exposure to respirable crystalline silica. According to the NPRM, which was published in the Federal Register, the PEL for respirable crystalline silica as an 8-hour TWA exposure is 50 g/m3. The American Conference of Governmental Industrial Hygienists (ACGIH) established a Threshold Limit Value (TLV) of 25 g/m3 for quartz and cristobalite (respirable fraction). The recommendation was made due to "concern about fibrosis (silicosis), the historical inflammatory process caused by silica exposures, and the association of inflammation and fibrosis with lung cancer," according to the TLV documentation. NIOSH document titled "Best Practice Engineering Control Guidelines to Control Worker Exposure to Respirable Crystalline Silica during Asphalt Pavement." 
Milling provides detailed recommendations for ventilation controls and water sprays on asphalt milling machines. The main points of the ventilation control recommendations are a drum housing enclosure, proper hood, and duct design, airflow capacity, duct, and fan durability, and precautions against ventilation control clogging. Manufacturers designed their systems to exhaust air and maintain negative air pressure in the milling machine's drum housing, which is where dust is generated, in order to contain silica dust. Milling machine manufacturers optimized water sprays along the primary and secondary conveyors in addition to the original water applications applied only to the drum to cool cutting teeth. Manufacturer A supplied and used an asphalt milling machine with dual diesel engines and a capacity of 534 kW (716 HP) that complied with EC Stage 3b / US Tier 4i emissions standards for the first 11 days at four sites. A 2 m (79 inch) wide cutter drum on an asphalt milling machine had ventilation control and dust suppression by water spray. By drawing air through a hood through the top cover of the primary conveyor closest to the drum housing using a fan and flexible hose system, the ventilation control created negative air pressure in the primary conveyor and drum housing areas. The secondary conveyor's end was where the ventilation control exhausted the air without filtration away from any workers. Additional water spray nozzles in the primary conveyor were installed to suppress dust along the conveyed path of recycled asphalt pavement, and water was applied to the drum housing to cool the teeth and suppress dust. The maximum water flow rate was 18 gallons per minute, but the operator could adjust it based on the milling operation's speed and depth. Manufacturer B provided an asphalt milling machine with a diesel engine that produces 462 kW (620 HP) at 1850 rpm and a cutter drum that is 2.2 m (86 inches) wide, which was used at 7 sites over the last 10 days. Manufacturer B's asphalt milling machine had a water spray system and ventilation controls, which included a 5.2 kW (7 hp) hydraulic Ilmeg fan connected to a 6-inch (15-cm) diameter duct leading to a manifold that divided the flow into two 4-inch (10 cm) diameter ducts that exhausted air at the top of the secondary conveyor. 
By drawing air through numerous slots in the top cover of the primary conveyor near the drum housing and close to the conveyor transition area, the ventilation control created negative air pressure in the primary conveyor and drum housing areas. The secondary conveyor's end was where the ventilation control exhausted the air without filtration away from any workers. The water spray dust suppression system included water applied to the drum housing as well as along the primary and secondary conveyors and 7 of NIOSH Publication No. 2015-105. The maximum water flow rate was 23 gallons per minute, but it could be changed depending on the speed and depth of the milling operation. PBZ air samples for respirable crystalline silica were collected from the milling machine operator and ground worker during normal asphalt pavement milling employee work activities. The milling machine operator would normally spend their shift on the operator bridge, which is located above the cutter drum housing. The operator is in charge of changing the controls for the milling machine's speed, steering, depth of cut, and water flow to the cutting teeth, as well as interacting with dump truck drivers using hand signals to fill trucks with recycled asphalt pavement. The ground worker would typically spend the majority of the shift at ground level, some distance from the drum housing, operating the ground controls on the milling machine's back sides. A ground worker's main responsibility is to operate controls on a milling machine at ground level that affect how well the road surface is cut. The ground worker may also be in charge of directing traffic around the machine, especially when it is milling through intersections, connecting the water truck's hose to the milling machine, and performing various other duties. The 42 PBZ air samples were collected over the course of 21 days at 11 sites and included a variety of conditions typical of asphalt pavement milling. Among the roads being milled were a parking lot, a major freeway, city streets, and rural highways. The milling depths included typical removals of one to three inches (2.54 to 7.62 cm) of the mill and fill as well as full-depth removals of up to eleven inches (28 cm) of recycled asphalt pavement. The sites that were chosen had a variety of daytime and nighttime milling. During the weather conditions, there was a wide range of ambient temperatures and wind speeds. The study included a variety of shift lengths, with the majority of shifts lasting between 8 and 12 hours. Respirable crystalline silica PBZ air samples were less than 50 g/m3 for the operator and ground worker across all 21 evaluated shifts covering a wide range of typical milling asphalt pavement conditions. For each occupation and manufacturer, the 95% upper confidence limit for the arithmetic mean exposure was less than 50 g/m3. This suggests that the average worker exposure for the population from which the sites were chosen is likely to be less than 50 g/m3. The findings show that when ventilation dust controls are combined with water sprays, a variety of typical highway construction tasks involving the use of asphalt pavement milling machines can be controlled in terms of occupational exposures to respirable crystalline silica. Following the testing period, the manufacturers of both asphalt pavement milling machines decided to make ventilation a standard feature on all new half-lane and larger asphalt pavement milling machines. Other asphalt milling machine manufacturers who are members of the Silica/Asphalt Pavement Milling Machine Partnership have agreed to conduct comparable field testing and install silica dust controls on their milling machines. 