There is a lot of conflicting information about whether or not pesticides are dangerous so it is no surprise that I get asked this question fairly frequently. I’d love to give a yes or no answer but the truth is, it varies depending on the pesticide in question and how you define ‘dangerous’. If you’ve ever wondered whether pesticides are dangerous, the following information should give you the information you need to answer the question for the specific pesticides you’re interested in.


What is a pesticide?

Before we can determine whether pesticides are dangerous, we first need to define what a pesticide is. A pesticide is any chemical, whether synthetic or ‘organic’, that kills pests. People often refer to insect pests when talking about pesticides but the term ‘pest’ can also encompass other organisms including pathogenic microorganisms and weeds.

Synthetic pesticides

Synthetic pesticides are those that are man-made. They tend to be more hazardous than home-made organic pesticides though not necessarily more hazardous than commercial organic pesticides.

Organic pesticides

In scientific terms, ‘organic’ means a material that is made from hydrogen, carbon and oxygen atoms. When it comes to organic pesticides however, the term refers to chemicals that are derived from nature rather than being man-made. Organic pesticides are usually made from plants but they can technically be made from anything that was once living. For instance, diatomaceous earth consists of the fossilised remains of diatoms (tiny, aquatic organisms)

Because they are made from natural materials, organic pesticides are often assumed to be safe. It is true that they are often safer than many synthetic pesticides but you should never assume something that comes from nature is ‘safe’. After all, snake venom is natural but you wouldn’t class it as safe. Similarly, some of the most toxic substances on Earth, such as botulinum toxin, tetrodotoxin, and ricin, are all produced by or derived from living things.

All organic pesticides are harmful to at least one organism (otherwise they wouldn’t be pesticides) but they are also often harmful to non-target organisms, such as humans and beneficial insects.

It’s also important to note that not all organic pesticides are approved to be used on organic farms etc. The chemicals that are allowed vary according to the certifying body but in general, only those that biodegrade rapidly are allowed.

What does dangerous mean?

In order to determine whether or not pesticides are dangerous, we also need to decide what we mean by ‘dangerous’. Many people define ‘dangerous’ as being hazardous to human health. Others however, are also concerned with whether or not a pesticide is hazardous to non-human life – specifically, whether it is hazardous to non-target organisms (organisms other than what the pesticide is designed to kill). The specific definition you subscribe to will determine whether or not a given pesticide is dangerous.

I’ll also note that some people will state that a pesticide is safe if humans are unlikely to ingest a toxic level of the chemical when eating a normal amount of food produced from plants that were exposed to the pesticide in question. This ignores however, any hazards associated with using a given pesticide. It also doesn’t take into consideration the negative affects of pesticides present in concentrations below the designated toxic dose.

So, are pesticides dangerous?

With all of the above in mind, we can now start to answer the question, ‘are pesticides dangerous?’.

Synthetic pesticides

Broadly speaking, many synthetic pesticides are hazardous to human health. They are often eye irritants and can be hazardous to breathe in. Some are hazardous if you get them on your skin though inhalation is often a bigger issue. Some synthetic pesticides, particularly older ones, are potent neurotoxins. Ingestion of the pesticides themselves is usually bad but the jury is still out on whether ingestion of the residues on food that was grown with the use of them is a big issue. Some scientists claim they’re completely safe at the doses found on food, however a growing body of evidence suggests that they may have more subtle negative effects at these low levels, such as a reduction in reproductive ability.

Many synthetic pesticides are also hazardous to other organisms as well. The more delicate the organism, the more likely it is to be affected by synthetic pesticides. Bees, for instance, can be affected by quite small doses of some pesticides. Many other beneficial insects are vulnerable to synthetic pesticides too. Aquatic organisms are also often vulnerable to synthetic pesticides if they get into waterways. As a result, it’s important not to use pesticides when rain is forecast (it’s a waste anyway as the application is likely to be less effective if applied before rain). Cats, dogs, and other pets are often vulnerable to synthetic pesticides so always keep these chemicals out of reach of your pets and do your research before using them in your garden. Other larger animals are often affected by pesticides as well. As an example, I was recently contacted by a person who had found a sick owl in their backyard. When she took it to a vet, the conclusion was that it was suffering from the side effects of a pesticide. The poor lady was horrified and promptly got rid of all the pesticides in her shed. It’s a sad example of what can happen when people don’t understand the risks associated with pesticides.

Some synthetic pesticides are persistent in soil and/or water which means if they’re misused (whether intentionally or accidentally) they can cause a lot of harm over a long period of time.

If you want to know whether a synthetic pesticides is dangerous, the best place to get some accurate (if not always complete) information is to check the product’s material safety data sheet (MSDS). There are sections for dangers to human health and for ecological hazards. Make sure you read the whole document because not all the pertinent information is in the summary at the beginning.

Organic pesticides

Broadly speaking, organic pesticides, especially those that are homemade, are not as dangerous as synthetic pesticides. This does not mean they are completely safe however. Many such pesticides are still hazardous to human health. For instance, pesticides based on chilli can cause a lot of damage if you get them in your eyes. Copper-based pesticides, which can be certified for use in organic agriculture, are not considered hazardous according to their MSDS but they can cause damage. For instance, skin contact may cause blistering, swelling or redness and it may be a mucous membrane and respiratory tract irritant. There is evidence to suggest that Neem oil might be harmful to or even terminate pregnancies. While some organic pesticides may be harmful if you ingest them directly, I’ve not found any evidence to suggest that it could be harmful to ingest their residues on food grown with the use of such pesticides.

It’s also important to note that quantity is important when considering whether a pesticide is dangerous. Synthetic pesticides are specially designed to target pests so often only a small amount is required for them to be effective. In a lot of cases, larger doses of organic pesticides are required to deal with a given pest and thus any toxicity is magnified.

As with synthetic pesticides, organic pesticides are also often hazardous to non-target organisms. In particular, the use of organic pesticides can still harm a wide variety of beneficial insects. Pyrethrum, for instance, is toxic to bees, fish and other organisms (including some pets). Copper, as another example, is toxic to earthworms and is very toxic to aquatic organisms. This is a very important point to note as a lot of people are under the mistaken impression that they can use organic pesticides as much as they want and they won’t harm beneficial microorganisms, their pets, or local wildlife.

Organic pesticides are generally (though certainly not universally) not persistent in soil and/or water so tend to do less damage if they’re released into the environment. Metal salts, such as copper fungicides, are one exception as they are not readily biodegradable.

When it comes to pesticides that are allowed inputs for certified organic farms, safety is often dependent on the certifying body. The USDA organic guidelines, for instance, are relatively lenient so larger quantities of more toxic substances tend to be used on US organic farms than in other systems. As another example, in Australia, there are a number of certifying bodies and their requirements vary greatly. In fact, one body basically only allows products that originate on the farm (so no external pesticides and fertilisers even if they’re certified organic – farmers have to produce their own fertilisers and pesticides).

If you want to know whether an organic pesticide is dangerous, check the MSDS if it’s a commercially manufactured chemical (or if there is a commercial equivalent). If it’s a homemade solution, you’ll have to do some broader research, especially about the base ingredients used to make it. Be careful to choose a reliable source of information as there is a lot of incorrect information out there. If you can get in touch with an organic pesticides expert, that would be your best bet.

Should I stop using pesticides?

That is a question you will have to answer for yourself. Personally, I won’t use any synthetic pesticides and always recommend that friends don’t use them either. I avoid organic pesticides as well though I have been known to resort to using a small number when I’ve had serious pest issues. As an example, I have in the past used a garlic and eucalyptus solution to reduce the numbers of harlequin beetles in our garden. Once we got numbers under control however, we stopped using that as well.

It may seem like pesticides are a necessity but I can assure you that there are perfectly safe and effective solutions for most pest problems that mean that you really don’t need to use any kind of pesticides most of the time.

Alternatives to pesticides

If pesticide alternatives sound like something you’d be interested in, here’s a brief overview of some of the options.

Firstly, you should know that unhealthy plants are always going to be more vulnerable to pest attack. This means a big part of pest control is making sure your plants are healthy. This, in turn, means ensuring they get enough water and nutrients. This doesn’t mean frequent watering and fertiliser application. Instead, if you need to feed your soil – provide lots of organic matter to boost water and nutrient holding capacity (and prevent evaporation) and to feed the beneficial microorganisms that will make nutrients from the soil and organic matter available to your plants. These beneficial microorganisms can also produce growth promoting exudates that will further improve the health of your plants.

Secondly, you can stop killing and/or disrupting beneficial organisms. Digging, fertilisers and pesticides can all harm beneficial soil microorganisms (see above point for why they’re important). Pesticide usage and poor plant choices also reduce numbers of or eradicate other beneficial organisms, such as predatory insects, that keep pest insects under control. The simple acts of companion planting (including growing flowering plants) and rotating your crops can have a massive positive impact on beneficial organism numbers and can result in you rarely (or never) having problems with pest insects. As an example, planting trap crops (when it’s done properly) is a very effective means of controlling aphids.

Thirdly, you can use techniques to deter pathogenic pests (fungi etc.). For instance, companion planting means you can pack in more plants while still allowing for plenty of air circulation (because you’re planting species with differing heights) which reduces susceptibility to fungi. Certain plants also actively inhibit some plant pathogens. If you are forced to irrigate, you can water first thing in the morning and you can take steps to avoid water splashing from the soil onto plant foliage. If you fail to stop an infection, sometimes a fairly benign organic pesticide can be used to treat it. For instance, an infusion of chives or garlic can help treat some fungal infections, especially if you get them early. Similarly, dilute milk can treat black spot.

When it comes to weed management, a bit of hard labour is usually highly effective even if it’s more effort than using pesticides. For large areas of deep-rooted weeds that are hard to remove completely, thick layers of porous, light-excluding material (such as newspaper) can be laid over the weeds. This excludes light, eventually killing the weeds, but doesn’t exclude water and oxygen, which are required to keep the beneficial soil organisms alive.

Finally, know that transitioning away from pesticides and synthetic fertilisers takes time. When you first reduce your use of these chemicals, you will probably notice an increase in pest problems because your garden hasn’t yet built up the beneficial organisms that will combat them in the future. Try to be patient and tolerant of the pests, knowing that in the end, you will get the better of them.

Examples of specific pesticides

To help you in your quest to learn about whether pesticides are dangerous, I’ve delved into a few commonly used pesticides below.


Glyphosate is one of the most ubiquitous synthetic pesticides used today so it’s a good example to use to illustrate the things to consider when deciding whether a pesticide is dangerous. Even if you’ve never used it before, you’ve probably been exposed to glyphosate as it’s commonly used by farmers growing food for human consumption as well as by many councils as they try to combat a variety of noxious weeds (just to name a couple of common uses). Organic produce also sometimes contains glyphosate residue as a result of pesticide runoff from neighbouring properties. A few companies sell pesticides containing glyphosate but Roundup is probably the most well-known.

So is glyphosate dangerous? This question gets a resounding ‘yes’. Even the manufacturer and distributors acknowledge this. For instance, the Australian MSDS for Roundup states that Roundup is “Hazardous according to the Globally Harmonised System of Classification and Labelling of Chemicals”. This means it is classed as hazardous to humans. The MSDS goes on to say that it is an eye irritant and is also hazardous to the aquatic environment. The MSDS also warns against inhalation of the substance.

In terms of ingesting glyphosate residue, according to Dr Ian Musgrave, a pharmacologist at the University of Adelaide, “you’d have to be eating 1,000 fold more than exists on current foods, or up to 100 kg of corn with residue every day to reach levels that are a hazard”. So it is considered safe from that perspective. Indeed, glyphosate replaced a range of much more toxic pesticides when it was introduced.

If you suffer from irritable bowel syndrome or any other digestive issue, you may be interested to know that there is some (animal) evidence to suggest that ingestion of glyphosate residue on foods might disrupt beneficial gut microorganisms (1, 2). As glyphosate acts by inhibiting a particular pathway used by plants and microorganisms to make certain proteins, it is not really surprising that ingesting even small amounts of it might kill off some of the bacteria in our digestive systems, which in turn is likely to cause digestive upset.

In the ‘ecological information’ section of the Roundup MSDS further information about toxicity to non-target organisms is given. It states that Roundup is toxic, to varying degrees, to rainbow trout, water fleas, green algae, duckweed, honey bees, earthworms, soil microorganisms (though it notes less than a 25% effect on nitrogen and carbon transformation after 28 days), bobwhite quails, and mallard ducks. This means, if you use glyphosate in your garden, there is a good chance you will harm something other than the weeds you are targeting. Note, this doesn’t mean that Roundup doesn’t affect other organisms. This just means that the above animals were tested and found to be negatively affected by Roundup. These animals are selected as indicator animals and can be used to predict whether a chemical will affect other similar animals. For instance, if rainbow trout are affected by a chemical, then it is likely that at least a few ‘higher order’ aquatic organisms will also be affected.

Interestingly, the MSDS states that it is not considered to be toxic to reproduction or a mutagenic or carcinogenic hazard. You may have heard that it is carcinogenic however so let me shed a little light on this issue.

An Australian cancer expert, Professor Bernard Stewart at the University of NSW said “residues in food are so low for all chemicals and glyphosate too, that there is no cancer risk”. This means that you are unlikely to get cancer from eating produce from plants grown in glyphosate-affected soil (at least if the glyphosate is applied according to the manufacturer’s instructions presumably). Interestingly, Professor Bernard Stewart doesn’t comment on the risk if you are regularly ingesting ‘Roundup ready’ crops. These are crops that have been genetically modified to resist the effects of glyphosate so that farmers can use greater doses of the pesticide to combat weeds without killing the crop plants. Food from such plants is likely to be contaminated with greater amounts of glyphosate residue. The situation is also a little murkier when you consider whether direct exposure to glyphosate can cause cancer.

There have been a variety of studies that have investigated whether direct glyphosate contact causes cancer. These studies have provided mixed conclusions. For instance, the World Health Organization conducted a review of the available literature and concluded that glyphosate is ‘probably carcinogenic to humans’ and a few studies have concluded that glyphosate exposure can increase the risk of non-Hodgkin lymphoma (3, 4). However, a study of more than 57 000 farmers didn’t find a connection between glyphosate and lymphoma and another recent review of the available literature concluded that there wasn’t a link between glyphosate and cancer in humans. This last point is an important one – most of the studies into carcinogenicity and toxicity are done in animals or human cells. Such studies give an indication of how humans will respond but are not conclusive evidence of an effect or lack of effect in humans. Obviously, it would be unethical to deliberately expose people to potentially harmful chemicals in order to determine the effects of those chemicals but it’s impossible to know whether effects in animals or cell lines will be replicated in humans. Another way of investigating toxicity is to study people who have been exposed to a chemical (through their work etc. – this is the ‘test group’) and measure the incidence of illnesses, diseases and cancer etc. in these people vs. people who haven’t been exposed to the chemical (the control group). The problem here is that any effects that are discovered aren’t necessarily due to the chemical being investigated. There might be something else common to the test group of people that causes the effect. And if no effects are noticed, it could be chance (if the test group is small, which is quite common) or the effects may take a longer time to appear (as is often the case with cancers). So as you can see, there are good reasons why various studies might not agree with each other.

To make matters worse, some studies have shown that the other ingredients in Roundup that increase the effectiveness of glyphosate also increase the toxicity of the Roundup solution (5, 6, 7, 8, 9). As such, studies that investigate the toxicity and carcinogenicity of pure glyphosate may be underestimating the effects of Roundup. A further issue is that many of the Roundup and glyphosate toxicity studies have been conducted by scientists that have a vested interest in either Monsanto (Roundup’s manufacturer) or an organisation that is against the use of glyphosate. As a result, we have no real way of knowing whether the studies were conducted in a completely unbiased way and thus whether the results are valid. This is particularly true of review studies as the authors may simply have chosen to review only those studies that support their agenda.

For those wondering how long Roundup hangs around for, the MSDS states that it adsorbs strongly to the soil and has a soil half-life of 2-174 days. This means that a year after using Roundup, up to nearly a quarter of the amount used may still be present in the soil. The MSDS does state however, that the half-life in oxygenated water is less than seven days. Thus, presumably, soils with more clay and less water flowering through them will hold onto Roundup for the longest. Whereas Roundup used in areas with sandy soil and high, frequent rainfall is likely to be removed from the soil more quickly.

So, is glyphosate dangerous? Yes, it is hazardous to human health as well as to other non-target organisms. Should you eat only certified organic produce that is likely to have fewer pesticide residues (though is not necessarily pesticide-free)? That’s up to you. You’re probably not likely to experience toxicity symptoms from eating conventionally grown produce but those with severe digestive issues may want to investigate whether eating home-grown, pesticide-free produce helps them at all and you may want to consider the growing body of evidence that suggests more subtle negative side effects to eat glyphosate and other pesticide residues. Plus, it may be too soon to determine whether such residues cause cancer given that we have not had hundreds of years of exposure to use in research.

Should you stop using glyphosate in your garden? To be honest, I wish everyone would stop using it because it can cause so much damage to the environment but again, it’s up to you. It is definitely dangerous if you or a member of your family (such as a child) accidentally ingest it or breath it in and it will damage the organisms in your soil, making it harder for you to grow healthy pest-resistant plants. It is also likely to harm a wide variety of the animal visitors to your garden. If these things bother you, don’t use it. You may even want to encourage your neighbours and friends not to use it. If you choose to use it, you should always follow the manufacturer’s directions and you should always wear safety gear when using it (such as gloves, goggles and a safety mask). You should also avoid using it on windy or raining days to minimise it being applied in areas other than where you intended. Never use it when there are animals about and try to avoid spraying it on insects. I personally won’t use it and discourage everyone I know from using it. I would strongly encourage you to use one of the very many alternatives at your disposal.


Neonics, a nickname for neonicotinoid chemicals, are a family of commonly used chemical pesticides that are related to nicotine. Imidacloprid, a member of this family, is probably the most commonly used insecticide around the world. Neonics are less toxic to birds and mammals than many older insecticides (though not safe for these animals) as they target the central nervous system of insects. If you’ve heard about them in the news, it’s probably because there is now a lot of evidence to suggest that neonics are very harmful to bees and that they are contributing to the worldwide decline in bee numbers. Since bees are largely responsible for pollinating a huge amount of the global food supply, this is a direct issue for humans (though of course, caring individuals would not want to cause the mass decline of bees regardless of the effect on humans).

Some countries have introduced bans on some neonics but every one of us can choose not to use these chemicals. Have a look at the pesticides in your garden shed and in your home (yes they’re in some fly sprays etc.) and check whether they contain neonics.

If the damage to bees and other non-target organisms doesn’t concern you, neonics are toxic to humans too, so yes, neonics are dangerous.


Strychnine is an organic pesticide (rats are a common target) and poison derived from many species of plants in the Strychnos genus. It causes muscles to contract uncontrollably which can eventually stop breathing. It is used in many countries to control wild animals so is unsurprisingly also toxic to a variety of other non-target organisms. This is one very dangerous organic pesticide.


Pyrethrum, or more accurately, pyrethrins, are extracted from the chrysanthemum daisy and are thus organic. Exposure to sunlight degrades these substances as does a change in pH. The degradation products are non-toxic, immobile in soil, and metabolised by soil microorganisms and thus they are allowed in organic operations. Note pyrethroids are synthetic versions of pyrethrins and they are more toxic and persistent than their natural relatives.

Pyrethrum and pyrethrins extracts are toxic to a wide variety of insects including bees and many other beneficial insects. It is also toxic to aquatic organisms. Pyrethrum can cause allergic reactions, long-term immune problems, asthma and other respiratory issues in humans but the dosage required for this is generally quite high. Pyrethrin extracts are less toxic. Some commercial pyrethrin mixes contain piper only butoxide. This chemical increases the insecticidal properties of the pyrethrins in the mix but also means the product cannot be used in organic operations.

Categories: GardeningScience

K. M. Wade

K. M. Wade is a multi-genre and freelance writer, gardener and scientist


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