Risk Assessment and Management

The U.S. Environmental Protection Agency (EPA)defines risk as “the chance of harmful effects to human health or to ecological systems resulting from exposure to an environmental stressor,” with a stressor being “any physical, chemical, or biological entity that can induce an adverse response.” Risk assessment, then, is a scientific process of quantifying and characterizing the nature of health risks to humans or to the environment, and risk management is the process which determines the best methods for protecting public and environmental health from these risks (EPA Risk Assessment website, last visited 30 April 2012).

Following the previous discussion on standards, it is clear that many of the same limitations to the development of adequate standards apply to the risk assessment process. Risks can only be characterized and quantified to the extent that we have scientific data available. As data are often either lacking or limited by available technologies, risk assessment and management are necessarily iterative processes. Moeller uses the nuclear accident at Three Mile Island as an example, as it helped to highlight deficiencies in previous risk assessment methodologies and led to improvements in the process.

There is also considerable overlap between the concepts of standards and risk assessment/risk management. In essence, risk assessment is an integral part of the process of developing standards, as it helps to determine and quantify threshold exposure levels. The development of standards, in turn, is part of the process of risk management, because the standards provide the necessary benchmarks for determining mitigation and enforcement needs.

The EPA lists related, but slightly different, steps for assessing risks to human health vs. to ecological systems. Both begin with what they refer to as the planning and scoping process, which is essentially the preparatory background research needed. For human health risk assessments, the steps are then hazard identification, dose-response assessment, exposure assessment, and risk characterization. In the case of ecological risks, the steps are problem formulation, analysis, and risk characterization.

In either situation, the risk characterization cannot be considered static, as new or ongoing research may reveal previously unrecognized effects, or even the presence of unrecognized confounders which have falsely inflated the apparent risk. In essence, the iterative process is that of the Plan-Do-Check-Act cycle of continuous quality control (ISO website, last visited 30 April 2012).The planning process and the assessment steps previously mentioned correspond to the Plan and Do phases of the cycle, respectively. The Check phase is one of determining how well the risk has been quantified/characterized and what deficiencies exist in the currently available data, and the Act phase is one of determining how to address these deficiencies for the next iteration of the cycle.

Although risk management is also an iterative process, it relies on far more than just the scientific and methodological considerations of risk assessment. Risk management must also take into account the feasibility of control measures, socioeconomic factors and political considerations. Additionally, risk management must factor in the potential impact of alternatives to the risk being considered. For instance, if the exposure of interest is the product or by-product of an industrial process, risk management must also consider the potential risks of pushing industry toward alternate processes, the product or by-products of which may pose equal or greater risks that the primary agent of concern.

Risk management is also not wholly dependent upon the existence of well-characterized or quantified risk. The Precautionary Principle is that of instituting control measures when there is evidence of serious damage to human health or the environment, even though the risk may not yet be well quantified. As the United Nations’ 1992 Rio Declaration stated, “[L]ack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.” This principle is, of course, highly subject to interpretation, and is not an acceptable substitute for a proper risk assessment. It does, however, provide a rational basis for instituting an initial risk management program until an apparent risk can be more accurately assessed.

Because risk management is also an inherently politically-charged process, as well, risk communication is an extremely important component of the process for developing consensus among shareholder groups, including the media, the general public, and policymakers. Although communication needs to be scientifically-based, it should also take into consideration the target audience, as non-technical audiences are likely to lose or misinterpret the intended message when highly technical language is used (FDA website, last visited 30 April 2012).

An example of a situation in which good risk communication was likely important in shaping policy was the passage in Maine last year of a ban on the sale in the state of reusable food containers made with plastics containing bisphenol A (BPA). BPA is used to harden some plastics by increasing crosslinking between polymer chains and is also used in the lining of food cans and in some types of paper. Although it has been known to possess some estrogenic properties since 1936, BPA was grandfathered in as an existing chemical under the 1976 Toxic Substances Control Act and industries were allowed to continue using it without safety testing (Flint, et al., 2012).

Although BPA does not bioaccumulate at a level that reaches the EPA’s threshold for concern, several studies have documented detrimental effects in some wildlife species at levels of contamination which are known to occur in the environment. These include the brown trout, which is an important game fish in the state. Additionally, there is growing evidence that it may increase infertility risks in women and may increase the risk of estrogen-responsive tumors, such as meningioma (Ehrlich, et al., 2012, and Duan, et al., 2012). Although this represents only a tiny fraction of the available data, the governor stated at a press conference that he didn’t see that there was any greater risk than women growing “little beards”.

Given that the governor’s party controls both branches of the state legislature and the fact that both branches voted overwhelmingly in favor of the ban, it is clear that the potential risks of BPA were communicated to them sufficiently to garner broad bipartisan support for the measure. Of course, there still remains the question of whether the ban will a) adequately reduce levels of BPA contamination, and b) avoid increasing levels of other, potentially more damaging, substitutes for BPA remains to be seen. With ongoing iteration of the assessment and management processes, though, and with good risk communication, there is at least some hope that these risks can be mitigated.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.

Flint, S., et al. “Bisphenol A exposure, effects, and policy: a wildlife perspective”. Journal of Environmental Management. 104(2012): 19-34. doi: 10.1016/j.jenvman.2012.03.021

Ehrlich, S., et al. “Urinary bisphenol A concentrations and implantation failure among women undergoing in vitro fertilization”. Environmental Health Perspectives. E-pub, http://dx.doi.org/10.1289/ehp.1104307

Duan, B., et al. “The relationship between urinary bisphenol A levels and meningioma in Chinese adults”. International Journal of Clinical Oncology. E-pub, doi: 10.1007/s10147-012-0408-6

Occupational and Environmental Standards

In his chapter on occupational, population, and environmental standards, Moeller starts by defining primary, secondary, and tertiary standards. Primary standards are threshold levels of exposure to chemical or physical agents, such as noise levels, that are designed to minimize the risk of human disease. Secondary standards are threshold levels of exposure designed to minimize damage to the environment or elements of human civilization, such as agricultural crops or manmade structures, or to meet aesthetic concerns, such as drinking water appearance and taste. Tertiary standards are derived standards intended to ensure that the primary levels are not exceeded.  Moeller points out that secondary standards are often more restrictive than primary standards, since some agents are undesirable even at levels that are below those known to have negative health impacts in humans.

One of the earliest exposures for which standards was developed was ionizing radiation, as its deleterious effects were recognized relatively early on in radionuclide research and utilization. Early standards were not formalized until the formation, in 1928, of the International X-Ray and Radium Protection Committee (now the International Commission on Radiological Protection, or ICRP) and the U.S. Advisory Committee on X-Ray and Radium Protection (now the National Council on Radiation Protection & Measurements, or NCRP).

Early standards were based on concerns about potential hereditary damage, derived from research done in Drosophila flies. From about 1960, though, standards were changed to reflect epidemiological studies conducted on survivors of the atomic bomb explosions in Hiroshima and Nagasaki, Japan. Those studies demonstrated no significant hereditary effects, but rather the effects of acute radiation poisoning and chronic health effects of exposure, particularly an increased incidence of leukemia.

Early EPA primary standards focused on the concept of the “maximally exposed individual”, with the idea that no member of the public should exceed such limits. Because such a hypothetical individual was difficult to identify, this evolved to the concept of the “reasonably maximally exposed individual” (RMEI), meaning the focus of limits was shifted toward critical groups of individuals with highest temporospatial risks of exposure -  eg., due to occupation, location, or lifestyle. By setting limits for such individuals, exposures should be kept below threshold for average members of the public.

Moeller lists eight criteria for establishing and applying standards:

1.       Understand the conditions for application of the standard and whether it is to be a primary or secondary standard.

2.       Identify what is to be protected – humans, wildlife, agricultural crops, buildings, etc.

3.       Review all possible pathways of exposure.

4.       Review applicable human epidemiological data.

5.       Review available data on negative impacts to nonhuman animals, plants, and the ecology.

6.       Establish a schedule for completing studies to fill in gaps in knowledge.

7.       Evaluate existing standards and their source.

8.       Develop guidelines for application and enforcement of the standard, including the limitations and uncertainties inherent in the standard.

The limitations of standards, of course, are manifold, and Moeller also lists several of these:

1.       The evaluated endpoint, usually a form of cancer, may have multiple causes, which can confound analysis of effects.

2.       There is very little dose-response data developed expressly for the purpose of developing standards, so much has to be extrapolated from other studies.

3.       Dose-incidence relationships are often poorly defined and can be complicated by differing risks for different routes of exposure.

4.       Standards often don’t account for differences between population groups – e.g., weight, size, diet, lifestyle, etc.

5.       There is often little or no coordination between standard-setting agencies, such as OSHA and EPA.

6.       Standards may not apply to all potential sources of exposure.

7.       Standards are generally set for a specific route of exposure and don’t take into account the cumulative effects of exposure via multiple routes.

8.       Sound epidemiological data for the effects of exposure may be lacking.

9.       The risks associated with a particular exposure may not be quantified.

10.   Few standards exist for the protection of the natural environment, property, or aesthetic concerns.

11.   Standards focused on maximal acute exposure may miss the effects of low level, chronic exposures, or biological effects may not occur in either a direct linear fashion or according the minimum threshold concept of effect – i.e., different effects may occur with different levels of exposure.

The two broad strategies for enforcing standards are a “top-down” and a “bottom -up” approach. In the first case, an upper bound is set and the limit reduced from that to a reasonably achievable level based on site-specific considerations. With the bottom-up approach, a relatively stringent lower level is set and then adjusted upward if needed due to feasibility or cost limitations. In theory, either approach should tend toward achieving a middle ground, but it’s not clear whether one or the other would be more subject to purely political considerations.  Because different agencies use different approaches, too, it is possible for incompatible or contradictory standards to exist.

Moeller suggests that there should be greater harmonization between federal agencies, in order to develop more uniform standards. At the federal level, I think this is a very desirable aim, as it should ultimately reduce bureaucratization. My sense, though, is that the current climate is one of minimal communication and coordination between agencies. Further, trying to coordinate standards at an international level is exceedingly difficult, and a uniform standard may not be desirable if it fails to take into consideration culture or region-specific differences.

That said, there are possibilities in this regard. The United Nations Framework Convention on Climate Change (http://unfccc.int/), despite contentions among nations, was able to institute the Kyoto Protocol, which set standards for greenhouse gas reductions for signatory nations. This has not, of course, been a completely successful effort, as Canada pulled out of the treaty and the United States Congress has never ratified the agreement, but it is an example of a widespread multinational effort at developing some uniformity of standards.

A smaller, but still very notable example, of multinational efforts at developing uniform standards is the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (http://www.ich.org/). This is a joint partnership between regulatory agencies in the U.S., the European Union, and Japan, as well as pharmaceutical industry, to develop more uniform standards for the drug approval process. This effort helps to streamline bureaucratic processes, but also has the advantage for industry of making the drug approval process more efficient, as testing for approval doesn’t have to be repeated multiple times to meet widely varied standards.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.

Electromagnetic Radiation

Electromagnetic radiation comes from both naturally-occurring and manmade sources and is generally classified as either ionizing or nonionizing, depending upon the amount of energy carried by the radiation’s wave particles, or photons. The energy level is directly proportional to and often expressed using the oscillation frequency of the photons. Ionizing radiation is that occurring in the high energy ranges – e.g., gamma rays, ultraviolet light, x-rays - which is capable of stripping electrons from atoms struck by the photons. The energy transferred in this reaction can cause cellular damage or other harmful changes in chemical or biological structures.

Nonionizing radiation, on the other hand, is that which occurs in lower energy ranges – generally from the lower end of the ultraviolet spectrum on down. Although generally thought of as safer, there are in some ways more questions surrounding the biological effects of nonionizing forms of radiation. Although we may know a great deal about the benefits and potential dangers of infrared radiation (i.e., heat), we are still learning about the effects of electric and magnetic fields.

Our understanding of ionizing radiation, which led to our understanding of atomic structure, dates back to the late 19^th century. Early experiments led to an understanding of natural sources of radiation, such as uranium and radium, as well as our ability to produce radiation via a high charge electrical field, such as is done today with diagnostic x-ray equipment.

And although there are benefits to be gained from the utilization of ionizing radiation – nuclear fission to produce electricity, x-rays for medical diagnosis, radioisotope therapy for some forms of cancers, etc. – there are also potentially harmful biological effects. When electrons are stripped from atoms within a living cell, the remaining anion is highly reactive and can trigger a series of chemical reactions which damage cellular structures and function. While some of this damage can be repaired, damage to DNA can result in cellular death or can result in changes that lead, sometimes several years later, to the development of cancer.

For this reason, exposures to manmade sources of radiation are typically tightly regulated. Within my own work setting, our x-ray and CT rooms are required to be lead-lined to prevent transmission of x-rays through walls. Personnel are required to wear lead-lined personal protective gear to limit exposure when we must be present in the room for imaging (which is most of the time when taking x-rays), and all staff who use the equipment must wear radiation dosimetry badges, so that our monthly and cumulative doses can be monitored.

Natural sources of ionizing radiation are somewhat more difficult to control and monitor. We are all bombarded with cosmic rays constantly, as these high energy rays can pass readily through buildings. People who fly frequently in commercial jets are exposed to even higher levels, since there is less attenuation by the atmosphere at higher altitudes. Here in New England, the large concentration of granitic deposits are also a terrestrial source for ionizing radiation, as they contain numerous radioactive isotopes, including uranium.

One of the by-products of radioactive decay in granitic substrates is radon gas, which can accumulate in basements with dirt floors (such as my own). Because radon is heavier than air and because it decays rapidly to a solid radionuclide, it can accumulate in the lungs and act as a source of long-term exposure, which can increase the risk of lung cancer. Because Maine has a fairly shallow soil layer overlying largely granitic rock, much of the state is classified as Zone 1 for radon exposure potential by the EPA, which means exposures here are among the highest (http://www.epa.gov/radon/zonemap.html).

Of course, we are almost all constantly exposed now to mandmade nonionizing radiation. Most houses are replete with electronic devices, such as computers, stereos, microwave ovens, televisions, etc. – all of which produce magnetic fields. Most Americans now own cell phones, many use Bluetooth devices to communicate through their phones, and cell towers are continuing to pop up all over the country to expand network coverage. All of these things increase ambient nonionizing radiation levels.

Cell phones, in particular, have been a source of considerable controversy in the last several years, at least since Hardell, et al. (1999) first posited a possible link between cell phone usage and brain tumors. The most recent report on the Interphone Study conducted by the International Commission for Non-Ionizing Radiation Protection Standing Committee on Epidemiology, however, found no association between cell phone usage and an increase in glioma or meningioma occurrence at this time. Although the authors did report some limitations with the study, they noted that the heaviest users were actually more likely to overestimate past usage, which would tend to bias findings toward a positive association (Swerdlow, et al., 2011). Additionally, these findings do not preclude the possibility that an association might become apparent in another 5 to 10 years or longer.

Perhaps the take-home message should be: Don’t be afraid to use your cell phone, but try to turn it off and walk away from time to time.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.

Hardell, L., et al. “Use of cellular telephones and the risk for brain tumours: A case-control study”. International Journal of Oncology. 15(1): 113-116. 1999.

Swerdlow, A.J., et al. “Mobile phones, brain tumors, and the Interphone Study: Where are we now?”. Environmental Health Perspectives. 119(11): 1534-1538. 2011.

Insects and Other Animals

As a veterinarian, a large area of my professional training and focus is in zoonotic disease, not only with regard to animal-to-human transmission but occasionally concerning the reverse, as well. Viruses will often mutate and jump from one species to another; bacteria are notoriously promiscuous and readily exchange DNA that will allow them to propagate more rapidly, often at the expense of host health; and parasites, though often more host-specific, can occasionally infect multiple species or can cause serious disease when they find their way into aberrant hosts.

Some animals can be very useful to humans, such as honeybees and other crop pollinators. Others are nuisance species and can cause serious damage to food crops, either by causing physical damage or by spreading viral or fungal plant pathogens.  Conversely, human activities can be damaging to animal populations, from habitat destruction to overuse of nonselective pesticides.

Because this topic covers such an enormous scope, I am going to focus primarily on areas in which I have particular knowledge and interest. First and foremost of these is rabies, which is one of the oldest recognized zoonoses. For the entirety of my career as a practicing veterinarian, I have lived and worked in rabies-endemic regions, principally the Atlantic seaboard states. The virus is primarily maintained in wildlife reservoirs and incidence fluctuates on a fairly regular cycle, as host populations wax and wane.

Because the disease is one of the earliest for which a vaccine was developed, vaccine programs are well recognized as the most efficacious means of prevention and standard protocols are well established. All fifty US states require vaccination of domestic pets, and approved vaccine products exist for livestock species, as well. As a result, human deaths from the disease are extremely low in the US, as well as in other countries which have adopted mandatory animal vaccination programs (i.e., essentially all developed nations). 

Worldwide, however, over 55,000 people are estimated to die each year from the disease, with the largest numbers of deaths occurring in Africa and in the Indian subcontinent (http://www.rabiescontrol.net).

In the US, the majority of human cases over the last couple of decades have been strains of the virus associated with bats. Mistakenly identified by Moeller as rodents (they are chiropterans), bats are a very important group of species ecologically, both as pollinators and seed dispersers and as consumers of insect pests. That said, they are also capable of finding their way into human dwellings through very small openings, and their bites are often not recognized, due to their small sharp teeth and the potential for victims to be bitten while they are sleeping or otherwise unaware of the bat’s presence (http://www.cdc.gov/rabies/bats/index.html).

Living in northern New England, tickborne disease are also a significant concern, especially Lyme disease, caused by the spirochete Borrelia burgdorferi. Lyme disease is not spread by direct transmission between affected species, but primarily by the tick species Ixodes scapularis. In clinical practice, I see the disease and identify the tick vector extremely frequently. There are some differences between the clinical syndromes in dogs and humans, but both can potentially experience polyarthritis, fever, and neuropathies. In dogs, we occasionally see immune-mediated damage to the kidneys, which generally carries a very poor prognosis. And although the organism is treatable, it is not considered to be a curable disease and recrudescence is possible.

When I lived in North Carolina, ticks were vectors for Rocky Mountain Spotted Fever, babesiosis, ehrlichiosis, anaplasmosis, and numerous other diseases. A study published one of my colleagues at NC State in 1999 showed high levels of coinfection in both dogs and humans at one kennel in the eastern part of the state (Kordick, et al., 1999).

Mosquitoes are also an important vector for disease spread. Worldwide, malaria is probably the most well-known mosquito-borne disease. Many other disease, including dengue fever and yellow fever, are also transmitted by mosquito species. In the late 1990s to early 2000s in the eastern US, mosquitoes were the vector for West Nile virus infection, which had spread to North America via migratory birds. Although the virus is host-adapted and rarely causes serious disease in definitive host species, in aberrant hosts, such as corvid birds and humans, it can potentially cause serious neurologic disease and even death.

Although syndromically similar, the western, eastern, and Venezuelan equine encephalitis viruses (WEE, EEE, and VEE, respectively), are genetically unrelated viruses which are also transmitted from birds to aberrant mammalian hosts via mosquito vectors. As their names suggest, they can cause neurologic disease similar to West Nile and can be fatal. My husband and I own alpacas, and the farm where most of our animals were boarding was one of several in southeast New Hampshire affected by an outbreak of EEE in the summer of 2005 (Nolen-Watson, et al., 2007). During that same period, a total of eleven human cases were identified in Massachusetts and New Hampshire, four of whom died from the disease (MMWR, 2006).

Clearly, control of vector and other pest species is important in managing these problems, but achieving this is not always straightforward. For some insect species, release of sterile males can help reduce population numbers, but given the short life cycle of most insect species, this requires sustained effort for long-term impact. The use of pesticides is problematic because of the risk of damaging non-target species – either beneficial insects or species higher up the food chain, such as occurred with DDT. In the case of rodent species, physical traps are not always humane or effective and poisonous baits can be consumed by non-target species – I frequently see rodenticide poisonings in pets.

That said, the challenges are not insurmountable. In malaria-endemic regions, selective spraying of DDT and increased use of mosquito netting has helped to impact the disease, which remains incurable. In agricultural settings, increased of integrated pest management strategies, such as promotion of predatory insect species, has helped to reduce the use of harmful pesticides. As long as we exist as part of the Earth’s ecosphere, though, we will have to continue finding a balance between our impact on other species and their impact on us.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.

Kordick, S.K., et al. “Coinfection with multiple tick-borne pathogens in a walker hound kennel in North Carolina”. Journal of Clinical Microbiology. 37(8): 2631-2638. 1999.

Nolen-Watson, R., et al. “Eastern equine encephalitis in 9 South American camelids”. Journal of Veterinary Internal Medicine. 21(4): 846-852. 2007.

MMWR. “Eastern equine encephalitis – New Hampshire and Massachusetts, August-September, 2005”. 55(25): 697-700. 30 Jun 2006.

Solid Waste

In 1976, Congress passed the Resource Conservation and Recovery Act (RCRA), which defined solid waste as follows:

“[A]ny garbage, refuse, sludge from a waste treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semisolid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations and from community activities.”

This act specifically excluded solids from sewage or industrial wastewater discharges, which were discussed as part of the liquid wastes journal entry.  A subsequent amendment to the act, though, also set a definition for hazardous waste, which is any solid waste or combination of wastes which may:

“(1) cause, or significantly contribute, to an increase in mortality, or an increase in serious irreversible or incapacitating illness; or (2) pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported, or disposed of, or otherwise managed.”

To address the matter of hazardous waste, Congress passed the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) in 1980, which has become more commonly known as Superfund. This act established regulations regarding the management of closed and abandoned hazardous waste sites, made parties responsible for hazardous waste releases at the sites liable, and established a trust fund for cleanup of sites when no responsible party could be identified.

In 1995, around 24,500 of the identified Superfund sites were designated as low priority and subsequently fell under the management of the Brownfields Development Initiative, which was created in 2002. This initiative provided government incentives for cleanup and redevelopment of these sites for public, private, or commercial usage.

Radioactive waste, such as spent nuclear fuels and some medical waste, is another category of solid waste, and mixed waste is, as the name suggests, a combination of two or more types of solid waste. And aside from the categorical considerations, solid waste management can be complicated by the fact that it encompasses items ranging in size from gum wrappers to retired airliners.

Management of hazardous wastes often involves some form of treatment "designed to change the physical, chemical, or biological character or composition" in such a way as to render the waste either nonhazardous or less hazardous and make it safer to store or dispose of. Methods of hazardous waste treatment include incineration, heat treatment (useful with volatile solvents), solidification into glass- or sand-like particles, chemical treatments to neutralize or degrade toxins, and biological treatments which use microbes to help break down or convert toxins into less hazardous forms.

Radioactive wastes are managed according to their potential level of hazard. Low level wastes, such as radioactive materials from healthcare or research facilities, pose a relatively lower risk and are generally managed by storage in earth-covered concrete bunkers with mechanisms designed to prevent leaching into surrounding soils. Uranium mill tailings are a particular high volume form of low level waste that are stabilized in situ by the Army Corps of Engineers. High level wastes, such as spent nuclear fuel rods or decommissioned armaments, pose a much higher health and environmental risk, and so the goal for managing these products is deep underground storage, though political considerations have made this challenging here in the US.

One particularly troublesome solid waste category over the past few decades is that of electronics. As computers and mobile phones have become increasingly commonplace, but with a relatively limited life span, it has become increasingly apparent that this portion of the waste stream will have to be addressed, especially given the fact that electronic devices typically contain numerous heavy metals and plastics which can have a significant negative environmental impact in quantity. Considering that an estimated 100,000 personal computers enter the waste stream daily, the need to deal with this problem is rapidly gaining urgency.

 And, of course, one major component of electronic devices is a major problem in its own right. To quote the movie The Graduate, “One word: plastics”.  In recent years, a great deal of media attention has been given to the “Great Pacific Garbage Patch”, an accumulation of plastic debris in the North Pacific Gyre. As documented in the video below, the effects on wildlife can be devastating, both due to mechanical injury, such as strangulation or bowel obstruction, and to toxicity caused by the propensity of plastics to adsorb chemicals, especially persistent organic chemicals such as dioxins and PCBs.

With regard to municipal wastes, landfills are the most common form of disposal. Because household wastes do contain some level of hazardous substances, landfills are regulated by the EPA under the 1984 RCRA amendments. They cannot be located in floodplains, on wetlands, near airports, or in areas subject to damage from disruptive geological events, such as landslides or earthquakes. They must be designed to prevent groundwater contamination, including clay and impermeable synthetic liners and leachate collection and treatment systems. Wastes must be covered daily to prevent spread of disease by or to vermin and wildlife, and access must be restricted to prevent unauthorized dumping. Monitoring systems must be in place to detect groundwater contamination or other problems, and lastly, operators must maintain monitoring systems for 30 years after a landfill is decommissioned.

Other methods for dealing with municipal wastes are surface impoundments, which are storage facilities that are generally ineffective and not adequately secure, and waste piles, which are non-containerized and which are primarily used for temporary storage. Some solids, such as sludge from municipal wastewater treatment facilities are managed by spreading them on the soil surface or working them into the soil. In this case, soil microbes complete the breakdown of waste components. Underground injection wells, into which waste is injected under high pressure, are only used in the US in certain exempt oil and gas wells.

Given the finite space available for landfills and the consumption demands of an ever-growing population, the demand for recycling has been growing, though many challenges still exist. The largest hurdle is, perhaps, developing a cultural imperative to recycle at all levels, from the individual to the municipal to the state and national levels. If we are to avoid ending up literally living on a junk heap, though, it’s a hurdle we will have to leap.

REFERENCES

Moeller, D.W. Environmental Health, Fourth Edition. Harvard University Press. 2011.