It appeared that a good number of people appreciate my diary from last Friday addressing the science behind pesticide risk and the utility of some pesticide use within reason. So for as long as I have ideas, I am going to do a bit of a running series on some of the scientific issues underlying problems with pollution and contamination.
Most people are aware of some of the common toxic metals, such as arsenic, lead, and mercury. Toxic metals (which are sometimes called 'heavy metals', although that term has no real scientific meaning) are some of the oldest known poisons, and are some of the worst and most common environmental contaminants. Toxic metals are particularly interesting and complicated as an environmental pollutant. Unlike most pesticides and industrial solvents, metals are not only naturally occurring, but ubiquitous. In addition, many toxic metals are also essential nutrients. They also have a maddening tendency to take on a dizzying variety of chemical forms and change as they make their way through the food chain.
So what are heavy metals, when are they dangerous, and what can we do about them?
Here is a general list of toxic metals:
Arsenic, lead, mercury, cobalt, copper, nickel, cadmium, chromium, molybdenum, zinc, manganese, vanadium, and titanium. (Of all these, mercury is the most unique and potentially most dangerous. I am not going to discuss this volatile little wonder here; quicksilver deserves its own diary).
Arsenic isn't quite a metal--it's on the border between metal and non-metal on the periodic table, so it's actually what's known as a metalloid, but it almost get lumps in with the rest of the toxic elements.
Now, let's clear up a confusion with terminology. The chemistry of metals is quite complex. People will usually talk about 'lead contamination' or 'mercury contamination', but they are actually addressing a whole host of compounds (sometimes referred to a 'species' or 'salts') that are formed with lead or mercury atoms and those of other elements (sometimes of other metals). Different forms of metals can have very different toxicological properties.
Toxicity is in the dose
Now, I hope that most of you noticed something very important about this list: turn around any typical bottle of vitamins, and several of these toxic metals are going to be listed on the back. What gives? Should you throw out your One-A-Days?
This question brings us back to the very important principle that I mentioned in my first environmental diary, and that always holds true when you are talking about issues of pollution: the toxicity is in the dose. There are many elements that can kill you at high doses that have no effect, or are even beneficial, at low doses. It's why you need to keep those Flintstone's out of reach of the little ones: we need iron, nickel, zinc, molybdenum (and maybe others) to survive, but large quantities are toxic.
(Interestingly, there are some compounds that have the toxic effect of interfering with absorption of toxic metals that happen to be essential nutrients. The most heart-breaking case my company has seen is one in which a family's farm--and livelihood--was wiped out by contaminants from municipal sludge that prevented their cows from absorbing copper, resulting in their deaths. Which seems strange on it's surface, considering that it is copper that is often a key contaminant at polluted sites).
This is a unique quality of metals among the chemicals that pollute. We don't need DDT. We don't need trichloroethylene. But we do need zinc. And honestly, nutrition science is still young; we may find that we need trace levels of lead, arsenic, even mercury.
Why is this? Unlike many chemical contaminants, metals are natural. They existed in the earth before human beings ever showed up. For a very lovely picture of what quantities of metals exist in the soil as background concentrations (i.e., the concentrations that cannot be attributed to specific point sources), take a look at these geochemical maps. (Two notes on these; one, not all of the elements mapped in this publication are heavy metals; two, this is a 12 MB file so it might take a while to download, but it is worth it).
Because metals have been around for as long as life has been on the earth, many organisms have found ways to use metals as nutrients. That includes many food crops, which selectively take up certain metals from the soil. It's good for us when it's broccoli taking up iron, since then we can eat that broccoli to get the iron we need. However, some plants take up the more toxic metals; potatoes like arsenic, and grapes like lead. So we can get metals exposure through eating food, even food from uncontaminated areas (I will discuss this more in the section on metals poisoning).
Do these plants, and other parts of the ecosystem, need lead and arsenic and other toxic metals? It is entirely possible that levels of certain metals that may be dangerous for humans may be beneficial to other organisms. Sometimes when evaluating a site for remediation, certain metals might be identified as contaminants of concern even though they are at background levels, since they reach the threshold of risk according to standard calculations. However, certain organisms in the area might be thriving with that concentration of the metal, even though it may pose a danger to humans.
Complicating things even further is the complexity of so-called 'background'. The maps I linked to are ostensibly 'natural' levels, but they do show a bit of anthropogenic noise. The high mercury levels around the Gulf, for example, are likely due in part to coal burning in the southern U.S. (many of the coal deposits in the south, especially in Alabama, are particularly high in mercury). Likewise, the elevated lead levels around major metropolitan areas are also likely anthropogenic. However, the very high levels of copper, chromium and nickel in Northern California are due to the geology of the area, and not to any actions by man.
So, untying what is natural, what isn't, and where the acceptable risk level is can be a complicated matter.
Metals contamination
Learning how to manipulate and use metals has been one of mankind's greatest accomplishments. We've been using metals for a long time, and eras of human history are even named according to their identification with specific metals--from the Bronze Age to the Steel Age. The relative ease with which metals can be molded, their high conductivity, and their durability have made them useful in a wide variety of applications, from electronics to buildings to dental filling.
We have used even the most toxic of metals. The word 'plumbing' is derived from 'Plumbum', the Latin for lead, since that is what they used for plumbing. Romans also used leaden pots to cook food and boil wine (the effect that chronic lead poisoning may have had on the Fall of the Roman Empire is hotly disputed: yea argument here, nay argument here). Chromium is used to make stainless steel. Many of the most highly toxic metals have long been used as dye--even as food coloring until the early 1900s (the notorious Red dye #2 has NOTHING on it's super-toxic predecessors).
I want to be clear--people have been aware of the toxic effect of these metals, even as they were using arsenic to make their apple jelly a sickly bright green. Even back in ancient Rome there were folks that were sounding the alarm about the dangers of lead pipes. Our knowledge of their toxicity is what led us to use so many metals-based pesticides, especially in the 19th and early 20th century. People were not as aware, however, of the effect of chronic poisoning--how steady exposure to tiny amounts might build up to a very significant exposure over time. More on that below.
Metals contamination can happen in a few primary ways. Metals can be released in waste streams from facilities that are using that metals as part of their operations--into the air, water, or as part of the facility's solid waste. Toxic metals are supposed to be handled as hazardous waste these days, but in the days before environmental regulation, metals-containing wastes were often disposed of without a lot of thought.
In the past, metals were also released through the proper use of products. The most noted examples are leaded paint and leaded gasoline. Leaded gasoline is largely responsible for the elevated levels of lead in the soils of cities, and remains a significant environmental threat. Paint containing metals--used either as a pigment or a pesticide--used to be common (some paints do still contain toxic metals, but in smaller quantities). Such paints, especially the previously common lead-based paints, can still cause a threat; as it dries and cracks it releases dust that can be inhaled. Also, children may eat paint chips, or are more likely to ingest dust through a hand-to-mouth pathway. Unfortunately, this problem can be especially bad in poverty-stricken areas, where old buildings have not been well-maintained. When you combine lead from old paint with the legacy elevated lead from gasoline, you can have very high exposures. The EPA has conducted some extensive lead abatement projects, but this is still one of the most significant environmental problems.
Metals contamination can also happen through the improper disposal of metals-containing consumer goods, such as electronics or fluorescent light bulbs. Sometimes, contamination can happen because people don't realize certain products contain toxic metals. 'Shredder fluff', the scrap metal that comes out of the car crushing process, is often contaminated with mercury because the shredders neglect to remove the mercury-containing switches that turn on the automatic lights in the trunk and glove compartments.
Some of the worst metals contamination doesn't happen because of intentional use, but because metals are released as a byproduct. Mining can inadvertently dump large quantities of metals that were previously bound up in ores into surface waters. Some of the most dangerous and complex Superfund sites in the country are mining sites.
Metals poisoning
We are frequently exposed to metals in small amounts; as stated before, some exposure is essential. A significant exposure can cause acute poisoning--such as eating a whole bottle of vitamins containing iron or working with metals in a work environment without proper controls. However, it is also possible to suffer chronic poisoning, from very small doses over a long period of time. How susceptible a person is to chronic poisoning may be partly dependent on their ability to excrete metals. In other words, some people may have more of a tendency to accumulate metals than others. The science on this is still very fuzzy.
Also, some different metals can have cumulative effects. For example, zinc exposure and lead exposure (even though zinc is generally less toxic than lead) can work together, because of the similarity of their effects.
People can have many low-level exposures through air, dust and through the food we eat. According to the very conservative risk equations that are sometimes used by regulators, however, very normal activities would incur an 'unacceptable' risk (sometimes quantified as a 1 in a million lifetime excess chance of an adverse effect). In California, following the passage of Proposition 65, wine and balsamic vinegar must be labeled as containing lead. In my personal opinion, this is nonsense. Exposures to these low levels of metals are just part of life, and if human beings were so sensitive I don't think there would be so many of us running around.
Poisoning does happen though, especially if someone works with metals or lives in a contaminated area. Children are especially susceptible, since they are small, still growing, and ingest about twice as much dirt as a typical adult. Chronic poisoning can have a host of negative effects on pretty much all of our systems. Common treatment for metals poisoning, either chronic or acute, is chelation therapy. Essentially, chelation uses drugs that help our bodies to excrete the metals. (I know more than I would like about chelation, personally, since my husband and my African Grey had to be on chelation therapy for a week after eating some grout in the bathtub that contained zinc--just part of the unending joy of parrot ownership. The therapy for the zinc was the same as for lead, again because of the similarity of these two metals).
Early chelation therapies were brutal; chelation has much improved. However, chelation has risks; it robs the body of nutrients that it needs, including calcium, and can kill you if not done right.
Unfortunately, there are a lot of quacks out there who play on people's fears about environmental contamination and try to sell them chelation therapy as a cure-all for everything from autism to fibromyalgia. As I said, there are definitely actual cases of metals poisoning and situations where chelation therapy is necessary. PLEASE don't fall for quacks, though, and always get a second opinion if someone is telling you that your problems stem from metals (unless you are having an obvious acute episode, of course).
Metals remediation
Cleaning up metals-contaminated sites is sometimes difficult. Metals don't break down in the soil the way many contaminants do. Often, the only thing to be done is to excavate the soil, and then treat all that soil as hazardous waste. This is extremely expensive and wreaks further havoc on the ecosystem. If a water body is involved, and there are metals in the sediments of a river, it causes even greater disruption to natural systems.
If the contamination isn't so dire, it can sometimes be preferably to plant 'hyper-accumulating' plants in the contaminated area to naturally clean the soil. It takes a long time, and you then have to be careful of how you dispose of the plants, but this is the kind of remediation I love, since it uses natural systems instead of tearing them up.
Conclusion
I am not advancing any particular opinion on metals (except that people who push chelation therapy for everything from heart disease to depression are quacks). I just wanted to address their particular complexity as an environmental contaminant, given the significant dangers that they can pose, our need for them, and their ubiquity.