Above: Sharp silica sand has angular edges that lock together.

What is silica?

Silica is a naturally occurring mineral found nearly everywhere on earth. It is the second most common substance found in the earth's crust. Silica is the common name for silicon dioxide, which is a compound formed from silicon and oxygen. Silica can exist in crystalline and non crystalline forms.

What are sources of silica?

Common products found at home containing or made from non crystalline silica include glass, cups, plates, bowls, pet litter, computer chips, bricks, concrete, plaster, paint, roofing, wallboard, and paper.

A common natural form of crystalline silica is quartz, but silica is also found in flint, agate, beach sand, sandstone, gravel, clay, granite and many other forms of aggregates and rock.

Because silica is everywhere, activities such as gardening, driving on road surfaces, farming, home construction, even kicking up sand on the beach cause silica to become airborne. Silica particles can be comparatively large - you can see them as dust - so this is not generally a health problem, although it may be classed as 'nuisance dust'.

Silica within hard rock is bound up and cannot be inhaled. However, when used in manufacturing or construction activities, quarrying or mining, silica dust is created as a by-product and this can be inhaled. It can then become a health issue if not correctly monitored and controlled.

Why is crystalline silica a health issue?

The size of the airborne silica particles is a crucial factor in contributing to a health hazard and determining the amount of risk. Larger silica particles are usually prevented from reaching the lung's small air sacs. It is the smaller particles - less than five thousandths of a millimetre or less than 10 microns (PM 10) - that are the most dangerous.

Above: An average human hair is 70 microns. Inhalable dust particle is 10 microns.

Breathing fine silica dust for prolonged periods can cause silicosis. The tiny particles lodge deep inside the lungs and cause irreversible scarring. In the old underground mining days many miners suffered from silicosis, also known as miners' phthisis.

Industrial exposure to crystalline silica

Today the most severe industrial exposures to very fine crystalline silica dust result from abrasive blasting, cleaning and smoothing irregularities from moulds, foundry castings and jewellery, finishing tombstones, etching or frosting glass. Other exposures to silica dust occur in cement and brick manufacturing, asphalt pavement manufacturing, china and ceramic manufacturing and the tool and die, steel and foundry industries.

In the maritime industry, shipyard employees are exposed to silica primarily in abrasive blasting operations to remove paint, oils rust or dirt from steel hulls, bulkheads, decks, and tanks before painting or treating. Workers in mining, stone cutting, quarrying (especially granite), road and building construction are at risk as well as workers in industries that manufacture abrasives. Farm workers are also at risk.

What about non crystalline silica?

Non crystalline silica particles are very small, but are not generally a health hazard. In fact non crystalline silica is used to purify water and control insect pests.

Microscopic single-celled plants that live in fresh or sea water, called diatoms, have intricate shells made of non crystalline amorphous silica. When large numbers of these shells are deposited, diatomite is formed. When diatomite is cleaned and packed to form a filter, the intricate geometry of the shells will remove impurities as small as 0.1 micron from water without the use of chemicals.

Diatomite can also be used as a non-chemical insecticide; the sharp silica shells cut and shred the insects. This insecticide is not harmful to humans despite sometimes being called 'sharp silica'.

Some equestrians use sharp silica sand as it 'locks' together to provide a firm surface. Under the microscope the corners of sharp silica sand are angular and defined like the edges of a cut gemstone. This is important in terms of how the sand moves under a horse's foot. The sharp edge grains of sand will tend to lock together and anchor the sand in location, resulting in firm footing that is not too deep.

During a recent inspection some improvements to the walkway were suggested and will be put in place in the next few weeks. The most noticeable will be a ramp at the western viewing platform to enable easier access for mobility scooters, wheelchairs and pushchairs.

Areas of plantings on the walkway recently received an overhaul. Approximately 40 - 50 large flax plants were thinned from two different areas: the KauriBank area at Grey Street near the noise bund, and the riparian zone along the Ohinemuri River bank at Clarke Street.

The plants were split and replanted in various locations on the walkway. The majority went to the wetland / Eastern Stream area in Slevin Park. Splitting and replanting the flaxes were undertaken in accordance with Maori flax planting traditions.

About 5,000 mostly native plants have been planted over the last few weeks and this work will be ongoing throughout July.