To understand lead toxicity, we need to know a bit about its chemistry.
Lead is a heavy metal that forms inorganic compounds (e.g. red lead, previously used in paint primers) or organic compounds (e.g. tetraethyllead, formerly used as an antiknock additive in petrol).
Organic lead is intensely toxic (80mg tetraethyllead would be fatal) because it crosses the blood brain barrier and directly affects the brain. Inorganic lead mimics calcium and fools calcium carrier proteins into transporting it into cells, but this is not as efficient as organic lead’s uptake and so inorganic lead is much less toxic (40g red lead would be near fatal).
Lead played a key role in the ascent of man. Its malleable and ductile properties and low melting point mean that it could be melted on wood fires, which made it ideal for forming pipes to carry water and making cooking pots. The Romans relied on lead to support their civilisation. They brought water into their communities through lead pipes, and they cooked food in lead pans and ate off lead plates. Not only did lead underpin their prosperity, it arguably led to their downfall.
Roman cookery relied on acresta (verjuice — juice from unripe grapes) which is acidic and dissolved lead from their cooking vessels. This, plus lead-contaminated drinking water, led to a significant lead intake. Indeed, recent archaeological studies show high lead levels in Roman bones.
It is likely that Romans gradually developed the central nervous system signs of lead poisoning: malaise, insomnia, loss of memory, delirium, convulsions. This did not augur well for continued success as a civilisation. Interestingly, Vitruvius (about 80-15 BC) realised lead was toxic: he said, ‘‘Water conducted through earthen pipes is more wholesome than conducted through lead ... ’’ But did anyone heed his advice? Clearly not. The Roman Empire collapsed.
Lead’s use continued for another thousand years, and we are still dealing with the Victorians’ penchant for lead water pipes and lead soldered iron pipes.
Lead causes anaemia, which can develop after short exposure (weeks to months depending on dose). It does this by inhibiting production of oxygen-carrying haemoglobin in red blood cells. This leads to fatigue, weakness and shortness of breath.
The life span of red blood cells is about 120 days — anaemia gradually develops as the haemoglobin-containing cells disappear from the bloodstream. Equally, when lead intake stops, it takes about 120 days for new haemoglobin-containing red blood cells to be replenished and anaemia to subside. Therefore, recovery from lead-induced anaemia is usually slow.
Long-term exposure to lead causes neurotoxicity. This is of particular concern in children because their central nervous systems are still developing and might be irreversibly affected. Lead’s neurotoxic effects are probably caused by a combination of toxic effects on nerve cells. For example, lead inhibits the release of some chemical messengers that carry nervous impulses between nerve cells. How it does this is uncertain, but calcium and magnesium ions have key roles in the process and lead mimics both.
Lead poisoning is treated using drugs like EDTA which bind to lead ions, carrying them out of the body in urine, but this is not effective for organic lead poisoning because EDTA does not cross the blood brain barrier or bind to organic lead.
The Latin name for lead, plumbum (hence its chemical symbol, Pb) is the origin of ‘‘plumber’’ because of their use of lead pipes. Indeed, the use of lead in water systems in Waikouaiti and Karitane might explain the high lead levels in their drinking water – lead solder was used to connect iron pipes in their water supply.
Lead has had many other uses, including in paint to increase durability and speed up drying. Its use in paints has led to dumps and landfills being contaminated and their runoff contaminating the environment. It is possible that runoff into the reservoir supplying Waikouaiti and Karitane could explain the high water lead levels. Lead in paint is important in children’s exposure because even though it has not been allowed in paints for many years, old paints still linger in some houses. The lead compound used in paint tastes sweet and this might encourage kids to gnaw on, for example, painted banisters. This might sound extreme, but I have seen several cases over the years. Lead shot from shooting might also find its way into reservoirs.
We do not know where the Waikouaiti and Karitane lead came from, but we know that the water levels spike, and that the spikes exceed the drinking water limit — the highest level was 394 ug/L which is nearly 40 times the statutory limit. This suggests an intermittent source of lead in the water supply. This might happen if water sits in a lead-containing system and is flushed out during water flow. This is just one way that lead levels could vary as much as they do in Waikouaiti and Karitane. Clearly, we must know the lead source if we are to tackle the problem.
The important question is, what health effects could exposure to lead at the levels found in Waikouaiti and Karitane cause? To answer this, we need to understand how the body deals with lead. I will focus on inorganic lead because this is the form present in the contaminated water. Inorganic lead is quite well absorbed following ingestion, but it is not well excreted and so it accumulates.
Long-term exposure to doses above the statutory limit might cause toxic effects — the higher the dose, the greater the toxicity. One-off exposure to a lead dose 40-times the statutory limit would probably have little effect, but repeat doses would. Intermittent doses would accumulate and might reach toxic levels. As the levels build up in the body, it might reach its toxic threshold and begin to cause anaemia or neurotoxicity. Therefore, it is not safe to drink tap water in Waikouaiti and Karitane until the lead source and extent of contamination are known.
The next issue relates to home-grown veges watered with lead-contaminated water: are they safe to eat? Lead is not taken up particularly well by plants, and tends to stay in the roots. Therefore, root vegetables often contain more lead than leafy veges. If lead-contaminated water is sprayed on to the vege’s leaves, the water evaporates and leaves lead behind. This is probably more of a problem than lead taken up by the plant. Even though washing well might remove this, it is better not to risk eating home grown veges watered with lead-contaminated water.
Finally, what do raised blood lead levels mean? This depends on two things, the level and its duration. We can’t predict toxicity from a single raised blood level but prolonged high levels require action. A single high blood level might reflect one-off exposure from food or another source not connected to Waikouaiti’s and Karitane’s water contamination.
Because long-term exposure to lead can have serious health effects, we must proceed very carefully and leave no stone unturned in our attempt to identify the lead source, monitor the population and treat them if they show prolonged raised blood lead. In the meantime, it is out of the question to drink the water. I prefer the precautionary approach: better safe than sorry.
