What is Light Pollution? Definition and Effects

light polluted sky, light pollution

When most people think of pollution, light pollution is not the first thing that comes to mind. Light pollution is one of the least talked about and least regulated forms of pollution.

Just over 100 years ago, most people in the world could look up and see a starry night sky, today approximately 80% of Americans can’t see the milky way. The cause of this decline in recent years is light pollution. This can be illustrated through a light pollution map.

Light pollution map of the United States
Light pollution map of the US, pink/red areas are the most light-polluted

This is a light pollution map of the United States. The bright areas represent the highest amounts of light pollution, they are usually around urban environments.

The definition of light pollution is the inappropriate or excessive use of artificial outdoor light. Like all pollutants, this has negative effects on human health, the environment, and the economy.

What are the types of Light Pollution?

Light pollution can be divided up into 4 main types of light pollution:

  • Glare – excessive brightness or unshielded lighting that causes visual discomfort. Think of this as someone shining a bright light in your face, it is painful for your eyes and blinds your vision.
  • Sky Glow – brightening of the night sky due to artificial light generally over urban and suburban areas. This can be seen in the orange/white light domes brightening the surrounding sky as you approach a city or urban area.
  • Light trespass – light falling into unnecessary areas. Think of this as someone putting up a light outside their house, but the light then falls into the neighbor’s yard and windows.
  • Clutter– groups of bright, confusing, and excessive light sources. The lights of Times Square are a classic example of this, countless bright lights are coming from all directions.

What Causes Light Pollution?

The cause of light pollution is the excessive use of artificial light at night, (people in the industry commonly use the acronym ALAN). These can be street lights, building lights, parking lot lights, front porch lights, and countless other artificial lighting sources.

According to a study by the National Park Service, 50% of the light from a typical unshielded light fixture is wasted, shining upwards and contributing to skyglow.[1]

10% is typically shining out horizontally producing glare, and only 40% is productive light, and of that 40%, some of that is usually light trespass as well.

When you start to multiply this by the number of light sources in any given area, the amount of light pollution begins to add up very quickly.

Image showing light pollution from an unshielded light fixture
Image credit: Chepesiuk R. Missing the dark: health effects of light pollution. Environ Health Perspect. 2009 Jan;117(1):A20-7. doi: 10.1289/ehp.117-a20. PMID: 19165374; PMCID: PMC2627884.

What are the Effects of Light Pollution?

At first glance, it may not seem like the effects of light pollution are that serious, so who cares if you can’t see the stars?

Light pollution has negative effects on human health including disrupting circadian rhythms, creating a cascading effect of health issues, it has negative effects on the ecosystem including disrupting animals that migrate at night, and it has negative effects on the economy, estimated to be approximately $3.3 billion, just in the US, in wasted light and energy.

How Does Light Pollution Affect Your Health?

Artificial light at night is relatively new to humans when looking at the issue from an evolutionary time frame.

Human beings evolved over hundreds of thousands of years with the natural light-dark cycle of the days and nights. Our circadian rhythms (our biological clocks) evolved to respond to this cycle. Electric lights have only been around for a little over 150 years and evolution is a slow process.

Light pollution affects people’s health by disrupting their circadian rhythms. The hormone melatonin is produced in response to darkness and when our circadian rhythm is interrupted, this process is interrupted as well.

Increased levels of light at night lower melatonin production. This decrease in melatonin can cause health problems including sleep disorders, fatigue, anxiety, headaches, stress, and some studies have shown a link between decreased melatonin levels and cancer.[2]

There are some steps we can take to mitigate this risk. Blue light in particular has the strongest negative effect on melatonin levels, blue light suppresses melatonin production.[3] You may have heard that you should use the night light setting on your phone and computer at night.

The night light setting shifts the screen to warmer colors, it will appear slightly orange. This will decrease the amount of blue light emitted by the screen.  

To limit blue light in your house, you should look for light bulbs with warmer colors. When choosing a lightbulb, usually the package will say daylight (blue-toned) or soft white (warmer-toned).

For lights you will be using at night, it is recommended to use the soft white bulb as that emits warmer colors and less blue light. In this age of artificial light, we need to protect ourselves from the effects when we can.

How Does Light Pollution Affect the Ecosystem?

Life on earth has been around for billions of years, and except for the last 150 years with the invention of the electric light, it has been uninterrupted with the natural light-dark cycle of day and night.

This cycle is the basis for everything including reproduction, migration patterns, sleep-wake cycles, and much more. Increasing sky brightness caused by light pollution disrupts this cycle and causes harmful and even deadly effects for many animals.

Light pollution negatively impacts many different animals. Many people who have stayed near the ocean are familiar with regulations requiring outdoor lights to be turned off and curtains drawn to shield indoor light at certain times of the year due to sea turtle hatches.

Sea turtle eggs are laid on the beach, and when they hatch, their instinct is to head towards the water which naturally appears brighter than the beach due to reflections from starlight and moonlight.

Artificial lights are interfering with this process, drawing the sea turtles inland towards the brighter artificial lights instead of towards the sea. These beach nighttime light restrictions have greatly helped with this issue.

Birds are also negatively affected by light pollution. Birds migrate using many different methods, and one of them is by using the stars. Light pollution can draw migratory birds off course and attract them to brightly lit urban areas such as cities where they face the hazard of tall glass buildings.

This problem is worse on cloudy nights as the birds’ view of the sky is obstructed and they get drawn toward the city lights. One stark example of this was the mass bird collision event that occurred in Philadelphia on October 2, 2020. More than 1,000 migrating birds were killed during one night within one small area of downtown Philadelphia in one night.

This prompted Philadelphia to join the National Audubon Society’s Lights Out program where buildings are encouraged to dim or turn off their lights at night during spring and fall bird migration to help prevent drawing birds towards cities.

How Does Light Pollution Affect the Economy?

Light pollution largely comes from light that is wasted and used excessively. This negatively affects the economy by the large sum of money being wasted to keep these lights on.

The International Dark-Sky Association (IDA) “estimates that at least 30 percent of all outdoor lighting in the U.S. alone is wasted, mostly by lights that aren’t shielded. That adds up to $3.3 billion and the release of 21 million tons of carbon dioxide per year! To offset all that carbon dioxide, we’d have to plant 875 million trees annually.”[4]

That’s a lot of money being wasted by residents, business owners, and local and state governments. The IDA estimates that “installing quality outdoor lighting could cut energy use by 60–70 percent, save billions of dollars and cut carbon emissions.”

There is definitely room for improvement with valuable savings involved.

Light Pollution Examples


How to Reduce Light Pollution

Light pollution is technically one of the most easily reversible forms of pollution. In 1977, New York City experienced a total blackout at night due to a power failure, and residents could see the Milky Way and other celestial objects over the city for the first time in around 100 years!

You can help reduce light pollution by using lighting only when and where it’s needed, by using properly shielded light fixtures, using outdoor lights with motion sensors or timers, and by advocating to your local government about the significance of light pollution and the importance of light-related ordinances.

There are many people that don’t know that light pollution is even a problem, so spreading the word will also help to advance the cause. You can join an organization like the IDA to help preserve our dark skies and get involved with light pollution advocacy efforts and education.

The good news is that every individual is capable of making a positive impact, and through these efforts, we can help to preserve our starry skies for future generations.

Help Support Light Pollution News!

[1] Chepesiuk R. Missing the dark: health effects of light pollution. Environ Health Perspect. 2009 Jan;117(1):A20-7. doi: 10.1289/ehp.117-a20. PMID: 19165374; PMCID: PMC2627884.

[2] https://education.nationalgeographic.org/resource/light-pollution

[3] Shechter A, Kim EW, St-Onge MP, Westwood AJ. Blocking nocturnal blue light for insomnia: A randomized controlled trial. J Psychiatr Res. 2018 Jan;96:196-202. doi: 10.1016/j.jpsychires.2017.10.015. Epub 2017 Oct 21. PMID: 29101797; PMCID: PMC5703049.

[4] https://www.darksky.org/light-pollution/energy-waste/