Microbicide Use – The Double-Edged Sword
By Susan Springthorpe, Centre for Research On Environmental Microbiology, University of Ottawa

Man has evolved in a world full of micro-organisms, many of which are unrecognized and unnamed, and only a small fraction of these are pathogens for humans. However, the ability for an unseen microbe to incapacitate or even kill a human instils a very instinctive fear of germs that is fed insatiably by the media and by advertising from manufacturers of microbicides. It’s only been within the last 15 or 20 years that there’s been a pressure to sanitize everything with chemicals, and I think that we have to ask if this is a good thing. During this time there has been no lessening in the emergence of new diseases and it is a period notable for increases in antibiotic resistance. I don’t need to tell you about the benefits of microbicidal chemicals. The intent here is to discuss the downside of microbicide use, the toxicity to humans and the environment.

If we consider all of the pesticides that are out there, there are more than 20,000 products, and many different active ingredients. Specifically for microbial control, the numbers are smaller – approximately 8000 products and 300 active ingredients – but just 14 active ingredients appear in more than 90 per cent of the available products in hospitals.

Microbicial chemicals are toxic and designed to kill, but unless you swallow quantities of them you are unlikely to get enough exposure for acute organ effects. We’re not talking about real, overt toxicity here.  The effects are much more subtle and they show up in most instances at the cellular level. The immune system and defence mechanisms are primary targets, potentially making those exposed more vulnerable to infections. Genetic and development effects are also possibilities from routine exposure. Discharge of these chemicals into water sources would be expected to result in similar cellular effects on species as diverse as small invertebrates, fish, amphibians and even mammals. There are examples of animals showing up with increased infections when exposed to toxic chemicals, though studies specifically linking microbicides with this phenomenon are lacking.

The additional toxicity of by-products that are formed during microbial use is extremely important. Microbicides produces disinfectant by-products because they are such reactive chemicals. These by-products can be more toxic than the original microbicide. This is the case with hypochlorite in drinking water, which has been very well studied. But other microbicide chemicals haven’t been examined in terms of risk from the by-products that they form.

The other thing that is really important is the changes in microbial populations following disinfectant use. Inevitably, you kill the easiest to kill organisms first, leaving you with the tougher ones, and if these happen to be pathogens the you have a problem. Moreover, there is evidence to suggest that bacteria surviving  sub-lethal exposures become more resistant on subsequent exposure. Clearly this is problematic for routine infection control measures, and illustrates the immense capacity of bacteria to adapt stressors in their environment.

In one study from California they identified four types of microbicides that were responsible for most occupational illnesses. These four chemical agents include hypochlorite, quaternary ammonium chlorides, chlorine gas (more confined to water treatment than regular hospital use), and glutaraldehyde. Hospital workplace exposure to microbicide toxicity would be mainly through skin and by inhalation. Patients might be exposed through active ingredient residuals on surfaces, as well as through inhalation and exposure to accidental spills. The noticeable problems that they cause typically present as hypersensitivity reactions, contact dermatitis, and in some cases asthma. Always these chemicals require very cautious handling and storage.

Quarternary ammonium compounds (quats) are legendary in that they are probably the most widely used microbicide in North America. One survey that we did many years ago found them to be in more than 70 per cent of registered biocides. Their primary level of activity is at the microbial membrane, where they tend to poke holes in the membrane and make them leaky. They quite often have somewhat lower human toxicity than hypochlorite or glutaraldehyde, but they are still sensitizers and you can find contact dermatitis and occupational asthma from exposure to these chemicals. Moreover, they are somewhat refractory to environment breakdown, although some bacteria can use them as a food source. They are used in such large quantities in institutional, industrial and consumer products that I think we will probably find them turning up in drinking water, when the water plant is downstream from municipal sewage outfalls. Non-chlorine oxidizers are tending to take over from quats in many cases; most notably hydrogen peroxide, and peracetic acid.

I have mentioned above the importance and problems of sub lethal exposure to microbicides, and this is a very important point to emphasize. Although none of us cares whether or not bacteria are exposed to toxins from the point of view of the health of the bacteria, those toxins are potentially changing those bacteria in much the same way that antibiotics change bacteria. They can induce resistance. Ironically, bacteria are often used to assess the mutagenicity (genotoxicity) of chemicals. The results are used to extrapolate how mutagenic they might be in humans. Nobody pays attention however to the effect that they might have on the bacteria themselves. We already know the problem that we have with antibiotics, which are after all just another toxin. We need to think about this very carefully and although there is little that can be done on this at the hospital level, we at least need to understand the issues.

To sum up: both humans and species living in water and soil environments are exposed simultaneous and sequential to pathogens and chemicals, and the risks are likely to be synergistic. There are many such chemicals involved, some of which are microbicides which get diluted as they are discharged in sewage. Such exposure is increasing with increasing prevalence of antimicrobial use. This is especially true healthcare settings, but domestic and other uses of microbicides are growing rapidly. The toxicology of many chemicals is inadequately recognized, and the possible potentiation of infections by them is poorly understood. There are major gaps in our knowledge of the combined effects and the real-life exposures to chemicals and microbes. Therefore, I consider that microbial control can create just as many problems as solutions. Strategies need to be put into rationalize microbicide use to those situations where the benefits can be shown to clearly outweigh the risks. It is particularly important to avoid the problematic exposure of pathogens to sub-lethal levels microbicides. Microbicides are therefore useful but they are dangerous and it’s a double-edged sword that we need to use with a great deal of care.

Excerpted from the Webber Training teleclass lecture, “The Human and Environmental Toxicity of Microbicidal Chemicals” (Apr. 3, 2008). For a copy of the lecture recording on Enhanced CD, contact Nicole Kenny at Virox Technologies ( This e-mail address is being protected from spam bots, you need JavaScript enabled to view it ).

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