Respirable crystalline silica: sources and health effects

Silica is everywhere, yet only some silica is dangerous and only in some forms. This lesson explains the difference between crystalline and amorphous silica, which particle sizes matter, where silica dust comes from at work and what it does to the lungs and other organs. The goal is to be able to look at a task and say whether it is likely to produce respirable crystalline silica, and to explain to workers why it matters.

Crystalline and amorphous silica

Silicon dioxide exists in two broad families. In crystalline silica the atoms form a repeating three-dimensional lattice. In amorphous silica they are arranged randomly, like glass. The difference matters because the crystalline forms are far more toxic to the lungs.

In practice, when workers and regulations talk about silica they mean respirable crystalline silica, often abbreviated RCS. Most occupational exposure is to quartz. Cristobalite matters in foundries, refractory work, kiln and furnace maintenance and some filtration media.

How much silica is in common materials

The amount of crystalline silica in a material strongly affects how much reaches the air when it is cut or ground. The ranges below are typical; individual products vary, and the safety data sheet or supplier should be checked.

Particle size: why respirable dust matters

The lungs have defenses against dust. Large particles are trapped in the nose and throat. Smaller particles that reach the airways are caught in mucus and moved upward by tiny hair-like cilia, then swallowed or coughed out. Only the finest particles reach the alveoli, the small air sacs where oxygen passes into the blood. There, particles are attacked by immune cells called macrophages, and this is where silica does its damage.

Occupational hygienists divide airborne dust into fractions by where it deposits:

For comparison, a human hair is roughly 50 to 100 µm across and a fine grain of sand is around 100 µm. Respirable particles are invisible to the naked eye except as a haze when present in large numbers. Exposure limits for crystalline silica apply to the respirable fraction, and air samples are collected with a size-selective device that mimics the lung's filtering.

How long does fine dust stay in the air?

Fine particles settle very slowly. Their settling speed in still air follows Stokes' law, which says that settling velocity rises with the square of particle diameter. A particle ten times smaller settles a hundred times more slowly. That is why respirable dust from a cutting task can linger in an enclosed space for hours and drift to workers who are not doing the cutting.

In the formula, v is the settling velocity, ρ is the particle density (about 2,650 kg/m³ for quartz), g is gravitational acceleration, d is the particle diameter and μ is the viscosity of air (about 1.8 × 10⁻⁵ Pa·s at room temperature). The short script below estimates how long particles of different sizes take to fall 2 meters in still air.

rho = 2650        # density of quartz, kg/m^3
g = 9.81          # m/s^2
mu = 1.8e-5       # viscosity of air, Pa.s
height = 2.0      # m, roughly breathing height to floor

for d_um in (50, 20, 10, 4, 2, 1):
    d = d_um * 1e-6
    v = rho * g * d**2 / (18 * mu)          # Stokes settling velocity, m/s
    t = height / v                           # seconds to fall 2 m
    if t < 120:
        when = f"{t:6.0f} seconds"
    elif t < 7200:
        when = f"{t/60:6.1f} minutes"
    else:
        when = f"{t/3600:6.1f} hours"
    print(f"{d_um:3d} um particle: settles at {v*1000:8.3f} mm/s, falls 2 m in {when}")

 50 um particle: settles at  200.590 mm/s, falls 2 m in     10 seconds
 20 um particle: settles at   32.094 mm/s, falls 2 m in     62 seconds
 10 um particle: settles at    8.024 mm/s, falls 2 m in    4.2 minutes
  4 um particle: settles at    1.284 mm/s, falls 2 m in   26.0 minutes
  2 um particle: settles at    0.321 mm/s, falls 2 m in  103.9 minutes
  1 um particle: settles at    0.080 mm/s, falls 2 m in    6.9 hours

Real workplaces have air currents that keep fine particles suspended even longer and spread them around. The practical lessons are that dust must be captured at the source, that people nearby are exposed too and that dry sweeping or blowing with compressed air puts settled dust back into the air.

Where silica dust comes from at work

Any process that breaks, abrades or disturbs silica-containing material can generate respirable dust. Common sources include:

• Construction: cutting, grinding, drilling and chipping concrete, brick, block and stone; tuckpointing and mortar removal; jackhammering; concrete floor grinding and polishing; dry sweeping and demolition.

• Stone and countertop fabrication: cutting, edging, polishing and installing natural and engineered stone.

• Mining, quarrying and tunneling: drilling, blasting, crushing and conveying rock.

• Foundries: sand molding, shake-out, fettling and cleaning castings.

• Manufacturing: glass, ceramics, pottery, brick and tile making; cement and concrete products.

• Abrasive blasting: especially where silica sand is still used.

• Oil and gas: handling frac sand at well sites.

• Agriculture and landscaping: tilling dry soils and cutting pavers and stone.

What silica does inside the lungs

When respirable crystalline silica reaches the alveoli, macrophages engulf the particles. The sharp, chemically reactive surfaces of the silica damage and kill these cells, which release chemical signals that attract more immune cells and stimulate the growth of scar tissue. The silica is released again when the macrophage dies, and the cycle repeats. Over time, small nodules of scar tissue form and can merge into larger masses. Freshly fractured silica, produced by cutting and grinding, is especially reactive.

Silicosis

Silicosis is the scarring of the lungs caused by crystalline silica. It is incurable and often progresses even after exposure stops. There are three recognized forms, defined by how quickly they develop:

Early silicosis may cause no symptoms. As it advances, people develop shortness of breath on exertion, a persistent cough, fatigue and reduced exercise capacity. In advanced disease, breathlessness occurs at rest, and the strain on the heart can lead to right-sided heart failure. Some people with severe silicosis need a lung transplant, and many die of respiratory failure.

Other diseases linked to silica

Silicosis is not the only risk. Respirable crystalline silica is classified by the International Agency for Research on Cancer as a Group 1 carcinogen, meaning it is known to cause cancer in humans.

Smoking and silica together are much worse than either alone for lung cancer and COPD. Workers exposed to silica who smoke should be offered help to quit, and health monitoring programs often include smoking cessation support.

Recognizing a silica task

A useful question for supervisors and workers is: does this task break, abrade or disturb a material that contains crystalline silica? If the answer is yes, assume respirable dust will be produced unless effective controls are in place. Lesson 4 explains how exposure is measured or estimated, and Lesson 5 covers controls.