Dust a post-earthquake hazard

DEMOLITION: Dust swirls around central Christchurch after the implosion of Radio Network house in August 2012.
DEMOLITION: Dust swirls around central Christchurch after the implosion of Radio Network house in August 2012.

Professor IAN SHAW looks at why demolition dust might be harming our health and explores whether the risk is worth worrying about.

I'm sitting in my office watching a building being deconstructed (the new euphemism for "demolished") with clouds of dust floating on a warm nor'wester towards my open window.

My first thought is "Close the window!", quickly followed by "I hope the deconstruction people have thought through the dust- associated risks."

So, what are the risks associated with dust and why has it become an issue? After all, we've been exposed to dust forever and it's done us no harm. Or has it?

Looking back into the annals of human health risk discoveries, the one that marks a significant change in thinking was Richard Doll's discovery in the 1950s that smoking is associated with lung cancer.

We all accept this now, but in the 1950s it was rather controversial.

If you look at the geographical mortality statistics for lung cancer in the UK it is clear that more people died of lung cancer in Lancashire, Manchester, Newcastle, Yorkshire and parts of Cornwall and Wales.

The common factor for these areas was mining.

We now know that breathing mine dust augments the development of smoking-associated lung cancer.

This was thought to be because chemicals in cigarette smoke cause the changes in DNA that predisposed a cell to cancer and that dust inflamed the lungs causing the cancerous cells to divide to form a tumour.

We still think this is the case, but we now know there is far more to dust than just inflammation.

Asbestos dust is a particularly good case to make this point. Asbestos causes cancer - a particularly horrible cancer called mesothelioma (cancer of the lining of the inside of the chest cavity).

To achieve its cancerous goal, an asbestos particle must be small enough to be taken deep into the lungs with inspired air - it must be about 5 millionths of a metre (5 µm) across or less.

When inhaled, the sharp asbestos fibre pierces the cells of the lung and causes changes in specific genes that control cell replication. But there is more. Asbestos fibres are inflammatory and cause cells to divide.

So, asbestos causes both changes to genes and increases cell division frequency - a double whammy - and this leads to uncontrolled cell growth and cancer.

The key features in this sequence of events are that the asbestos dust particle is small enough to penetrate deep into the lungs and that it is sharp enough to penetrate a cell.

Asbestos is a well-known dust carcinogen and is taken very seriously by building "deconstructors".

But what about dust from other building materials like, for example, concrete? Does this have the same effects on lung cells as asbestos? Is it a health risk?

The reason, of course, that we are asking these questions is because during and following the Christchurch earthquakes we have been exposed to very much more dust than during those halcyon pre-earthquake days.

I remember well looking back at Christchurch from the Port Hills following a tortuous journey home on February 22, 2011 to see a huge dust cloud floating over Christchurch and since then every building demolition has released more dust.

There's another thud outside my office as another bit of the adjacent building falls to the ground in a cloud of dust while a man with a hosepipe tries to dampen down the dust cloud.

Studies in workers exposed to concrete dust have shown that crystalline silica is the problem and that it is associated with a broad array of lung diseases, including silicosis and lung cancer.

Crystalline silica, like asbestos, has particles small enough to inhale deeply into the lungs. Chemically it is silicon dioxide and occurs in rocks and minerals naturally (such as quartz).

Concrete consists of crushed rocks (aggregate - often silica- based) held together by cement. When concrete is ground or crushed it produces fine respirable particles, including silica derived from the aggregate. Therefore when a concrete building is demolished it inevitably produces crystalline silica dust.

There's been a lot of demolition in Christchurch and so there has been much exposure to this lung toxin over the past three years. The question is what, if any, effect will it have on our health?

The effect is dependent on dose. From studies overseas, the incidence of lung diseases (such as silicosis) is greatest in concrete and stone workers who grind the materials.

Grinding, of course, creates a great amount of dust and puts these workers in the top exposure bracket.

At the bottom of the exposure continuum are the general public who are exposed sporadically when they are in the environs of dust generation (such as demolition).

A bit higher up the exposure spectrum are people like me who sit in offices with demolition close by for weeks or even months. The worst effects (such as lung cancer) appear to occur only after high level, long-term exposure, but more of this later.

It is very difficult to link exposure to effect, but data from the UK clearly show that there has been an increase in incidence of mesothelioma ("asbestos cancer") with time and that the incidence is very much greater in men.

This suggests occupational exposure; however, women's mesothelioma incidence has increased too, albeit to a very much lesser extent, which suggests general environmental exposure to asbestos is also a factor.

The level of exposure determines the effect. The lung- related effects of exposure to concrete dust range from reduced lung function to cancer.

So, at the top end of the exposure spectrum (such as people who are exposed repeatedly to concrete dust over a long time), lung cancer is a risk that must be considered seriously.

For people exposed to less dust less often, silicosis is the key risk. And those exposed to low levels for short periods (such as people living or working close to demolition sites) might experience reduced lung function.

This is a very simplistic view because other factors are important in determining the effects of exposure to concrete dust.

Smokers are certainly at very much greater risk of harm because the effect of inhaled crystalline silica and cigarette smoke is greater than the effects of the individual exposures - it is synergistic.

Similarly, people with respiratory diseases (such as asthma) are likely to have a more pronounced response to inhaled concrete dust.

So, in the big picture of life's risks, living in and around Christchurch near to demolition work is adding an extra hazard to our lives which increases our health risks.

For most of us, this is a very small increase in risk, but for demolition workers the risk is very much greater.

Clearly, exposure to the hazard - concrete dust - should be managed to reduce the risk.

There are other, less worrying but toxic, aspects of demolition dust that we should not forget.

Depending on the materials from which the building is made, there might be toxic metals (such as cadmium) in the dust or toxic hydrocarbons adhering to the dust surface.

All of these will find their way deep into our lungs with the dust.

Their effects are unknown, but most certainly won't be good. While much less pressing, this is another reason not to breathe demolition dust.

From what I have seen, attempts to reduce "deconstruction" dust are variable and quite often ineffective.

If this was a student project I was marking, my final comment in red ink would be: "Must do better!"

Ian Shaw is Professor of Toxicology at the University of Canterbury.

The Press