How exposure to lead impacts human health

We thank the author Peter Franklin, Assistant Professor in population health at University of Western Australia and The Conversation  for permission to publish under Creative Commons.

Editorial Comment
Lead poisoning has to be added to the many health hazards arising from the more extreme floods occurring with climate change. The ABC Radio National program Lead Poisoning; a silent epidemic, describes how the wave of home renovation after the floods in Queensland resulted in lead paint stripped from houses increased exposure to lead in childhood. This is an excellent program with a list of references.

To the long list of infective conditions secondary to flooding it is apparent that we must add mobilisation of chemicals and heavy metals from the environment. The floods caused a washout of herbicide into coastal waters and the discharge of heavy metals from flooded coal mines into river catchments.

What is lead poisoning?
It’s exposure to lead in a way that lands the metal into your body. There are two types of lead poisoning – acute and chronic. Acute poisoning will occur with higher exposures, while chronic poisoning will generally result from low exposure over a longer time, and the accumulation of lead in our bodies (either in the soft tissue or bone).
Acute lead poisoning leads to symptoms such as headaches, fatigue, digestive problems, vomiting, diarrhoea or constipation. It also leads to convulsions, loss of consciousness, muscle weakness, and, at the very extreme, death, but it’s very unusual these days.

Acute poisoning may result from inhalation of lead dust or fumes, for example, lead soldering without protective equipment or for young children in particular, ingestion of contaminated soil or old lead paint. Exposure also depends on the type of lead (inorganic or organic) and its bio-availability. Both inorganic and organic lead can be toxic but will be absorbed at different rates and via different types of exposure.

Chronic poisoning is when lead accumulates in the body over time. A lot of it can be excreted, but some is stored in soft tissue, such as muscle and organs, as well as hard tissue, such bones. More lead is stored in bone than soft tissue.
Chronic exposure has been associated with neuro-behavioural effects in children. It can also cause muscle weakness, fatigue, general malaise and other non-specific symptoms. It’s been linked to cardiovascular effects, such as increased blood pressure in adults, although there’s some uncertainty about this.

How are we exposed to lead in our environment?
Prior to it being phased out, a lot of exposure was through the lead in petrol and in paint. Lead in petrol was phased out during the 1990s and early 2000s and levels in the air have decreased in that time. Lead exposure can be a problem in areas where the metal is mined or transported.

Lead is a heavy metal and a persistent compound, so it’s also found in soil. Soil exposure is always an issue, particularly around where lead has been mined, transported or along vehicle corridors where it was previously emitted from exhausts. Although levels have reduced over time, lead can still be found in the soil in cities.
It’s also not in paints anymore, but the problem comes up during renovations because older paints had lead in them. Soil and old paint contamination becomes a problem for young children because they have a lot of hand-to-mouth movement and that means a lot of ingestion. Young children in particular are very tactile and may chew on a wooden cot with old paint on it, eat soil, or put their dusty hands in their mouth a lot, so it’s always an issue for them.

And there are periods of vulnerability, with research showing the early period is very important. There maybe another period of vulnerability in the early school age. Researchers found that when they held everything else constant, exposure at early school age was associated with significant neuro-behavioural problems later in life.
There’s also some concern about imported toys that might contain lead or lead-based paint. It’s disallowed here but products from abroad have been a problem in recent times.

What is its impact?
Probably the biggest interest has been in neuro-behavioural effects, such as the effect on IQ. Adults usually get occupational exposure although, in theory, they should be using protective equipment to prevent this. Children are affected more than adults because they tend have higher exposure, particularly through ingestion. They also absorb it quicker and retain more in the body than adults.

Lead exposure has been linked with ADHD and anti-social and even criminal behaviour.
Lead poisoning decreases IQ in what may seem a small measure for an individual – a few points – but that’s a big issue from a population point of view. The problem is that it isn’t obvious. You don’t get people suddenly not being able to read. It’s not like a respiratory epidemic, where everyone’s going around wheezing and coughing, it’s just happening in the background.

Is there a safe level of exposure to lead?
There are guideline levels for blood and for different media – so for soil levels, water levels and food, but there’s no good evidence of a threshold below which health effects don’t occur.

There are no safe levels of exposure, but there are guidelines for what’s allowed. How does that follow?
Obviously guidelines are required but we should not just use them to assume that reaching that level is good enough. For compounds such as lead, where there seems to be no threshold level for health effects we should be aiming for zero exposure. That’s unlikely to happen but we can’t use guidelines just to say everything’s okay. Guidelines should be re-set or adjusted as the evidence becomes available.

It’s a difficult field. As soon we find evidence that we’re below a “safe” level and think the war is being won, we find more health effects. What makes it hard to understand is that the impacts aren’t obvious. You just can’t see them in day-to-day life – a one-point or 1% drop in IQ for an individual is not easily spotted. And this is particularly difficult in children because you’re just following their progression and they change at their own rate. You can’t really say they could have progressed better.

But small changes in individuals can have a large impact at the population level and it’s important to keep using data from big population-based studies.

What can be done to mitigate exposure?
Removing lead from paint and petrol was a a good, and necessary, change. For residential areas, away from mining and transport hot spots, both airborne and soil levels of lead are normally low, and probably decreasing.
But there areas that still have high lead concentrations. These are mining areas and transport hubs, such as Mount Isa, Port Pirie, Broken Hill, and they’re often in the news because of potential problems of lead poisoning. Such places obviously deserve special consideration in mitigation and control and it’s an ongoing concern. Many readers are probably aware of the issues in Esperance a few years ago, where transporting lead to the port caused bird deaths and high levels of lead in the blood of children.

In such places, regulations and mitigation strategies need to be very strict and there needs to be ongoing monitoring.

The evidence is that once damage – particularly neurological damage – is done, it can’t be reversed. So where damage results in a significant impact on biological mechanisms it won’t be reversed, but we can obviously try to reduce ongoing exposure.

To reduce blood lead levels, primary prevention such as reducing routes of exposure is the best strategy. Chelation therapy is used for lead poisoning but I don’t know if it’s effective.

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