Pollution comes in many forms, from plastic pollution to chemical substances entering our waterways. But one of the biggest forms of pollution is pollution of the air. This is a growing global environmental problem which comes in many forms and therefore causes many different direct and indirect problems. We now know many of these pollutants are having adverse effects on our own health and so this raises the question that, if air pollution is so bad for us, what are the effects of air pollution on animals?
In short, air pollution affects animals both directly and indirectly and the amount it affects a certain animal will vary greatly depending on a number of factors. Direct effects include:
- Inhalation of gases and particles in the air
- Ingestion of particles in food or water
- Absorption of gases through the skin
Indirect effects include:
- Climate change
- Ocean acidification
- Acid rain
- Ozone layer depletion
As you can see there are plenty of potential effects of air pollution on animals. Some of these are quite well understood and some are not. In this article, I will briefly try and summarise what we know.
What is an air pollutant?
A substance that causes air pollution is known as an air pollutant and is defined as:
“a substance in the air that can have damaging effects on humans and the wider ecosystem”.
This could be a solid particle, such as dust or soot emitted from a coal-fired power station. But it could also be a gas, invisible to the naked eye, such as carbon dioxide (CO2), the most well known ‘greenhouse gas’.
It is important to note that for a substance to be an air pollutant, it doesn’t have to have originated from human activities. Natural air pollutants also occur such as when a volcano erupts, or a forest fire begins naturally, this has been a natural process for a long time before humans were affecting animals (we are just making things much worse!).
The difference between primary pollutants and secondary pollutants.
Any substance directly emitted and instantly having adverse effects are primary pollutants. Secondary pollutants are caused by these primary pollutants reacting with another substance causing a new pollutant to form.
Tropospheric ozone or ground level ozone is an example of a secondary pollutant that forms near the earth’s surface when other primary pollutants react.
What are the main air pollutants?
Before we can understand why pollutants can have adverse effects on animals we need to have a quick rundown of the main culprits.
Carbon Dioxide (CO2) – gas
I’m sure you have all heard of this one. The most well known of all the greenhouse gases (https://en.wikipedia.org/wiki/Greenhouse_gas).
Abundant in the atmosphere already, but thanks to us humans, now in far greater quantities following the industrial revolution and the rise in factories and vehicles emitting the gas at an alarming rate.
Nitrogen Oxides – gas
Another group of pollutants which includes nitrous oxide (N2O).
Nitrogen and oxygen are commonly found particles in the atmosphere which don’t react together at normal temperatures. However, under extreme temperatures (such as those found in a power plant or a car engine) they do and are released into the air.
Nitrous Oxide is actually a greenhouse gas with 298 times the ability of CO2 when it comes to trapping heat (over a 100 year period). However, it is currently in much lower quantities than CO2 and so is only thought to have 1/3 the effect on climate.
Sulphur Dioxide – gas
This is another one you may well have come across. Sulphur dioxide does not float around in the air like CO2 however it does enter the atmosphere naturally during volcanic eruptions.
However, we have been adding significantly to the quantity of this pollutant by burning coal and petroleum to produce power.
Methane (CH4) and other Volatile Organic Compounds (VOCs) – gas
Another group of pollutants that contribute to the greenhouse effect.
Methane directly and the other VOCs with their ability to form ozone which makes methane last longer in the atmosphere.
Some of the main human-made sources of methane are decomposing food/ waste in landfill and also from animal agriculture (farting cows basically).
Carbon Monoxide (CO)
You may (and if not probably should) have a carbon monoxide detector in your home. This is because carbon monoxide is a colourless, odourless and yet extremely deadly gas and can be formed when burning natural gas (such as in an oven).
The main man-made source of CO though is within vehicle exhaust fumes.
Unlike the invisible gases mentioned above, airborne particulates are very small particles of solid or liquid that are light enough to be suspended in the air for a length of time. Soot or dust are common examples of this type of pollutant.
These are produced naturally from volcanoes and forest fires and then by humans when we burn anything, such as in power plants.
So far I have mentioned some of the main offenders, but there are many more air pollutants on top of that including:
- Chlorofluorocarbons (CFCs) produced by refrigerators and aerosols
- Ammonia from farm waste
- Free radicals from burning fossil fuels
And the list goes on. But now we know the main air pollutants that we as humans have contributed towards over the years what are the effects of them on animals?
Direct effects of air pollution on animals
In humans, the direct health effects of air pollution are becoming more well known. With respiratory disease, heart disease and lung cancer among the worst.
The air pollutants responsible for these diseases are the airborne particles but also the nitrogen oxides emitted by road vehicles.
But as we are only really just beginning to understand the effects on humans the effects on most animals are still not that well known.
The effect of air pollution on pets, which spend much of their time in the same environments to humans, you would expect to be similar.
And studies have found adverse effects on dogs. A study in Mexico City, a city with notoriously high levels of air pollution, compared dogs living in the city with dogs living in an unpolluted area. They found that dogs in the polluted areas had inflamed brains and other signs indicative of disease. (https://www.sciencedirect.com/science/article/abs/pii/S0278262608001747). But there are still very few studies on the subject.
Indoor pollutants are also bad for pet animals with a study on cats showing a decrease in lung capacity when sharing a home with a smoker (https://www.sciencedaily.com/releases/2007/08/070831123420.htm). So when a study showed that on a day in Beijing with particularly high pollution, standing outside is the equivalent of smoking 1.5 cigarettes an hour (http://berkeleyearth.org/air-pollution-%20overview/). From this, we can infer that when air pollution is bad it must surely have an adverse effect on these animals as well.
Arguably even more at risk to the direct effects of air pollution are birds. Unlike pets, they don’t spend time indoors and are often resident in our cities where air pollution levels are high.
A study in the early 1990s examined the effects in quite an extreme way by placing caged birds (coal tits and rock buntings) within close vicinity of a working coal-fired power plant. The mixture of particulates, nitrous oxide and sulphur dioxide was shown to have adverse effects on the respiratory system of the birds (https://link.springer.com/article/10.1007/BF01061089).
More recently in 2017, a paper compiled a series of studies of air pollutants on birds dating back as far as 1950 and found consistent evidence of adverse effects. Effects included respiratory illness, affected immune systems, changes in behaviour and less success in laying eggs and therefore producing young (https://iopscience.iop.org/article/10.1088/1748-9326/aa8051).
The same study mentioned above where birds were placed in cages by a coal-fired power station was repeated on small mammals (wood mice and house mice). Yet again the pollutants caused respiratory problems.
A study in Sao Paolo, Brazil, (another city with high levels of air pollution) placed mice outside in cages for 4 months. One cage with filtered clean air and the other with unfiltered polluted air. The study found that when exposed to the unfiltered air from an early age, the reproductive success of the females went down (https://www.ncbi.nlm.nih.gov/pubmed/15820725).
The indirect effects of air pollution on animals
This is where it starts to get complicated. It gets complicated because everything in the environment is inter-connected making it very hard to pinpoint exactly how these pollutants are indirectly affecting animals.
If air pollution is affecting just the tiniest of microbes, that could still have indirect implications for other animals further up the food chain. So I won’t try and explain every single impact in this article but I’ll highlight a few of the more interesting/ concerning.
Indirect effects caused by climate change
As mentioned earlier in the article, many of the man-made air pollutants contribute to the greenhouse effect. Acting as a blanket in the atmosphere trapping heat in. The more of these gases we release the worse this effect becomes.
The warming effect caused by this is what we refer to as ‘global warming’ although because the earth’s climate is a complex system of weather patterns, climate can be effective in many different ways and all these can affect wildlife.
The number of studies linking climate change to impacts on wildlife is increasing all the time. The key thing to remember again is that everything is linked and so if climate change starts to affect even the smallest plants or insects, it will also more than likely affect the largest animals eventually.
Many species have evolved together over millions of years, with certain insects relying on certain plants to survive. If those plants are impacted by warming temperatures the insects will suffer and then the animal which eats the insects may also suffer, you get the picture. It doesn’t take much, a slight change in the life cycle of one animal can completely ruin that of another if they can’t alter to be in sync.
One of the main effects of climate change that has been observed is a change in range, i.e where certain animals are found. A study in Britain of 329 different species showed that 275 moved further north, most likely due to the warmer climate (http://www.theurbanclimatologist.com/uploads/4/4/2/5/44250401/hicklingetal2006insectsfishpoleward.pdf).
Rising ocean temperatures are causing corals, which are the foundation of huge ecosystems, to ‘bleach’. This is where corals expel algae living in their tissues leaving them looking white or ‘bleached’. This doesn’t kill the corals straight away but does lead to a higher rate of mortality. (https://oceanservice.noaa.gov/facts/coral_bleach.html)
The list of impacts is vast and ever growing. But as I say the important thing to note with all these indirect impacts is the fact that the slightest change to one plant or animal species can affect many others.
Other indirect effects of air pollution on animals
Ocean acidification occurs when the increasing amount of CO2 from air pollution is absorbed by the oceans. The absorption of CO2 changes the pH of the seawater i.e makes it more acidic.
As with climate change if this change occurred slowly, like it might have done before humans arrived, then animals could potentially adapt to the different conditions. However, these changes are happening rapidly thanks to all the air pollution we are causing and animals just can’t adapt quickly enough.
This more acidic water makes it difficult for animals to create shells, and has actually been found to start dissolving the shells of existing ones https://royalsocietypublishing.org/doi/full/10.1098/rspb.2014.0123.
The acidic water has also been found to alter other important functions of reef fish such as communication and reproduction (https://www.sciencedirect.com/science/article/pii/S0169534712002625).
Acid rain is formed when sulphur dioxide and nitrogen oxide air pollution combines with water in the atmosphere causing it to become acidic. When this water falls as rain it can then have some bad consequences for wildlife.
The clearest effects of this are on aquatic animals in streams, lakes, swamps and marshes. The acid rain has been found to react with soils to release aluminium which washes into waterways. This aluminium causes acidification and has effects on the survival of fish eggs for example.
The more acidic water also causes more mucus buildup on the gills of adult fish. This affects their ability to absorb oxygen and they can suffocate. This has lead to some pretty apocalyptic scenes of dead fish floating on the surface of ponds and lakes.
More acidic conditions have also lead to impacts on frogs, snails and other species. Frogs are slightly more resilient to acidic conditions than fish, but the shallow ponds they use to breed can be more rapidly affected by acid rain which quickly makes it impossible for frogs and other amphibians to breed in them.
Ozone layer depletion
Ozone (O3) is a molecule in the atmosphere that occurs naturally and forms a layer, known as the ozone layer, that acts to absorb potentially harmful UVB rays from the sun. Those rays that cause us to tan (or burn) although too many of which would cause us to be covered in skin cancer.
Chlorofluorocarbons (CFCs) are an air pollutant released by refrigerators and aerosols. These are broken down in the atmosphere to form, among other things, chlorine. The chlorine reacts with the ozone and this converts it to oxygen which no longer provides the protection from those UVB rays.
So what does this mean for wildlife?
Well it has been shown to affect the development of plants, which you now know will consequently affect many animal species too. It has also been shown to have effects on marine life https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2427.2003.01036.x.
More recent findings of the effects of air pollution on animals include:
Some air pollutants (such as ozone) have been found to interact with and break down the scent molecules produced by plants that bees use to find their food. This increases the amount of time they have to spend foraging and therefore causes a decrease in average lifespan (https://linkinghub.elsevier.com/retrieve/pii/S1352231016305210).
Another study showed that plants in urban areas with high levels of nitrogen dioxide produced more chemicals to help defend themselves against insects that want to eat them. https://www.nature.com/articles/s41467-018-07134-9
The list goes on.
Negative feedback cycles
Just to make things look even bleaker I need to mention negative feedback cycles. These are effects that are triggered by the things I have already mentioned that make things worse and worse.
One example is that as temperatures on the earth rise due to climate change ice begins to melt at the earth’s poles. Now as you know ice is white, and as you will also know a white surface is very good at reflecting sunlight. You might notice this on a hot day if you touch a black car and a white car, the black car will be hotter as it has absorbed more heat, whereas the white car will be cooler as it has reflected it. This is known as the ‘albido effect’.
Once ice has melted you are left with the earth’s surface or an ocean exposed underneath. Neither of these surfaces is as good at reflecting the heat of the sun as ice and so less heat energy is reflected back, more is absorbed and the earth heats up even more. Hence the term ‘negative feedback cycle’.
The list of the effects of air pollution on animals just keeps growing, as scientists carry out more studies and we see the effects of a longer time period (remember man-made air pollution is still a relatively recent phenomenon).
The fact is that the direct impacts of these substances are bad enough for many animals, but the indirect effects are making things even worse.
As I mentioned already animals and ecosystems have adapted to changing conditions on earth for millions of years. But those changes have mostly been slow. And when they have been rapid, such as when a large volcano erupts, this has lead to mass extinctions.
Since we started burning fossil fuels and producing motor vehicles, changes have been rapid and animals simply can’t keep up.
It is therefore clear that the effects of air pollution on animals is very bad indeed and I’m sure further studies will show those effects to be even worse than we think now. The quickest solution is to stop our pollution outputs as quickly as we can.
We have started to do this with some success in some areas. Bans on CFCs in some countries have led to a halt of ozone layer depletion and we are seeing the harmful UVB radiation reduced as a result of that. But for some other pollutants such as CO2, because the effects perhaps aren’t so immediately obvious to humans, the desire to stop producing them is not enough.
We need to work hard to find solutions and to change policy to stop the effects of air pollution before it is too late. If not for ourselves, for the animals.