Reversible versus irreversible toxic responses
Toxic responses differ in their eventual outcomes; the body can recover from some toxic responses, while others are irreversible. Irritation of the upper respiratory tract by inhaled formaldehyde gas, for example, is rapidly reversible in that as soon as the inhalation exposure terminates, the irritation subsides. In contrast, the response produced by silica dust is irreversible because, once the silicotic nodules are formed, they remain in the alveolar region of the lung.
The toxic effects produced by a toxic agent may be reversible or irreversible. Reversible toxic effects are those which disappear soon after the exposure ceases and the toxic agent is detoxified and eliminated. Irreversible toxic effects on the other hand, persist even after the exposure is discontinued. Effects like carcinomas, mutations, damages to neurons and liver cirrhosis are obviously irreversible as they are usually produced by permanent damage or changes in the tissue systems.
There are some toxic materials which cause reversible effects when administered in low concentrations and irreversible toxic effects when higher concentrations are introduced in the system. The chances of recovery from exposures which cause irreversible effects are very low and they are considered very dangerous.
Certain toxic effects are considered irreversible though they disappear after the exposure has ceased. Insecticides which inhibit the activity of enzyme cholinesterase for short duration which is approximately the time required for the synthesis of the replacement enzyme are examples of this type. The toxic effect is considered irreversible because the enzyme once affected is made useless. The recovery is actually due to the formation of fresh enzyme.
The toxic affects produced by a toxic agent may appear immediately after the exposure or there may be some time gap between the exposure and the appearance of toxic response. For example, Cyanide poisoning the toxic effect occurs immediately after the exposure, while some effects, particularly those caused by the interference of toxic agent in the synthesis and/or function of nucleic acids and proteins may appear months or years after the actual exposure, or at times they may appear in the following generation. To determine the delayed effects of toxic agents on a living system low term studies are essential. Such affects are considered to be the most dangerous ones.
Bio-accumulation in a biological system
Some contaminants that enter biological systems are preferentially stored (usually in fat tissue) in organisms resulting in an accumulation over time. This process is called bioaccumulation (also biomagnification) – see image at right.
An organism at the base of a food web may contain low levels of a contaminant. Its consumer, however, will concentrate the contaminant as it consumes many individuals of its food source over its lifetime. With each step in the food web, these contaminants become increasingly concentrated as more are ingested and stored. The amount of contaminant accumulation is greater in food webs with more steps to the top predator.
Therefore, the top predator in systems with longer food webs usually has higher contaminant concentrations than those with shorter food webs, all else being equal. The accumulation of a substance, such as a toxic chemical, in various tissues of a living organism: the bioaccumulation of mercury in fish. The uptake and retention of substances by an organism from its surrounding medium (usually water) and from food. However, it is commonly taken to measure the uptake over time of a substance, called a bioaccumulant, that can accumulate in a biological system.
Bioaccumulation can be divided into bioconcentration and biomagnification. Bioconcentration considers uptake from the non-living environment while biomagnification describes uptake through the food chain. For many fat-soluble and persistent organic pollutants (POPs), biomagnification is the dominant factor. Substances in biological systems are routinely excreted, degrading, reacting into something different and so forth. Most substances have a short “half-life”, as they are metabolized or excreted as waste. However, some compounds may stay in a system for a much longer period of time. For example, calcium in the human body is laid down in bones and teeth, and even when bone cells die, their calcium is used again in the building of bones.
If the input of a substance to an organism is greater than the rate at which the substance is lost, the organism is said to be bioaccumulating that substance. Thus, inter alia, the longer the biological half-life of a toxic substance, the greater the risk of chronic poisoning, even if environmental levels of the toxin are very low. This is one reason why chronic poisoning is a common aspect of environmental health in the workplace. As people spend so much time, for so many years in these environments, very low levels of toxins can be lethal over time. Naturally produced toxins can also bioaccumulate. The marine algal blooms known as red tides can result in local filter feeding organisms such as mussels and oysters becoming toxic; coral fish can be responsible for the poisoning known as ciguatera when they accumulate a toxin called ciguatoxin from reef algae.
References
Bryan, M. Waldichuk, R. J. Pentreath and Ann Darracott (2005). Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.
Stadnicka, J; Schirmer, K; Ashauer, R (2012). Predicting Concentrations of Organic Chemicals in Fish by Using Toxicokinetic Models. Environ. Sci. Technol.
Jon Arnot et al. (2009). Molecular size cutoff criteria for screening bioaccumulation potential: Fact or fiction?
Ashauer, R; Hintermeister, A; O’Connor, I; Elumelu, M, et al. (2012). Significance of Xenobiotic Metabolism for Bioaccumulation Kinetics of Organic Chemicals in Gammarus pulex. Environ. Sci. Technol.
