Skip navigation

2. What is Respirable Crystalline Silica?

What is Respirable Crystalline Silica Presentation

Key Points

Respirable crystalline silica (RCS) is a form of the mineral silica. Silica is formed of the two most abundant elements in the earth’s crust; silicon and oxygen. Each silica molecule consists of one atom of silicon and two of oxygen giving a chemical formula of SiO2.

Naturally occurring silica can exist in two forms:

-          Amorphous

In the amorphous state the silica molecules form a random pattern rather than being in an ordered state. The amorphous form of silica is quite rare in nature. An example of naturally occurring amorphous silica is diatomaceous earth.

-          Crystalline

In the crystalline state the silica molecules form a three dimensional repeating pattern. The crystalline state of silica is the most common form in nature.

The second most common naturally occurring mineral is crystalline silica in the form of quartz. Other less common forms include tridymite, crystobalite and semi-precious and gem stones such as agate and opal.

Crystalline silica is found in every type of rock with igneous (volcanic) rocks such as granite containing up to one third quartz, sedimentary rocks such as sandstone up to 80% quartz, and sand and silt up to 95% quartz.

This course is about crystalline silica in its’ respirable form. Respirable crystalline silica (RCS) is the type of crystalline silica dust that is of the most importance when assessing the risk to the health of exposed workers.

Respirable dust consists of very small particles of dust that can penetrate into the deep parts of the lung. This deep part of the lung is called the alveolar region and is where the blood takes the oxygen from the air we breathe. These particles are less than ten millionths of a metre (10μm – ten microns) in diameter. This is less than one seventh the diameter of a human hair. 

Fig 1

Now complete the multi-choice questions below to test your understanding.

Select the correct answer for each of the following questions.


What are the two elements that make up silica?


Hydrogen and Oxygen

Iron and Silicon

Silicon and Oxygen

Hydrogen and Silicon



The most common naturally occurring form of crystalline silica is?








Individual respirable particles are:


Half the width of a human hair

Less than 10 millionths of a meter in diameter

Visible with the naked eye.

Not able to penetrate into the alveolar region of the lung.