We are surrounded by an invisible killer. It is so common that we hardly notice it, even though it shortens our lives.
It causes heart attacks, type 2 diabetes, and the latest research even links it to dementia.
What do you think it's about?
The answer is noise.
And its impact on the human body is much broader than hearing damage.
"This is a public health crisis.
"A huge number of people are exposed to noise every day," says Professor Charlotte Clarke of St George's University, London.
But that's the crisis we're not talking about.
So I researched when noise becomes dangerous, talked to people whose health is at risk, and tried to find out if there was a way to cope with a world full of noise.
I started by visiting Professor Clark in the eerily quiet acoustics lab.
We're going to test how my body reacts to noise, and I'm wearing a device that looks like a large smartwatch.
He will measure my pulse and the amount of sweat.
You can participate too, if you have headphones.
Pay attention to how these five sounds affect your feelings.
AUDIO: Listen to five different sounds in less than a minute - how do you feel?
The sound that bothers me the most is the traffic noise in Dhaka, the capital of Bangladesh, which holds the title of the noisiest city in the world.
I immediately felt like I was in a huge, stressful traffic jam.
The sensors register my anxiety - my pulse rises sharply, and I sweat more.
"There is very clear evidence that traffic noise has a negative impact on heart health," says Professor Clarke as she prepares the next sound.
The only thing that has a calming effect on my body are the playful children's voices from the schoolyard.
Barking dogs and a neighborhood party in the early morning hours trigger a negative response from my body.
But why does sound affect my body?
"Sound evokes an emotional response," explains Professor Clark.
The ear detects sound and transmits the signal to the brain, where the amygdala, an area of the brain responsible for emotional responses, evaluates the feeling.
This is part of the body's "fight or flight" response, which evolved to help us react quickly to sounds like the rustling of bushes as a predator approaches.
"So your heart rate goes up, your nervous system gets activated, and stress hormones are released," Professor Clark tells me.
All of this is useful in emergency situations, but in the long run it starts to cause harm.
"If you're exposed to noise for years, your body is constantly reacting in this way, which increases the risk of heart attack, high blood pressure, stroke, and type 2 diabetes," explains Professor Clark.
This insidiously happens even while we are fast asleep.
You may think you are adapting to the noise.
I thought that too when I was living in a rented apartment near the airport.
But biology says otherwise.
"You can never turn off your hearing; even while you sleep, you are still listening."
"So these reactions, like a rapid heartbeat, also occur in sleep," adds Professor Clark.
Noise is unwanted sound.
Transportation - vehicles, trains and planes - are the main sources of noise, but so are the sounds of our entertainment.
While someone enjoys a great party, for someone else it's unbearable noise.
I meet Coco in her fourth-floor apartment in the historic neighborhood of Villa de Gracia in Barcelona, Spain.
There's a bag of lemons hanging on her door that a neighbor gave her, a tortilla in the fridge that another neighbor made, and she offers me some delicious cakes made by a third neighbor who's taking a pastry course.
The balcony offers a view of the famous Sagrada Familia church.
It's easy to understand why Coco fell in love with life in Barcelona, but she pays a huge price for it and thinks she will be forced to move away.
"It's extremely noisy... the noise goes on 24 hours a day," he tells me.
There's a dog park nearby where pets "bark at 2, 3, 4, 5 in the morning," and the courtyard is a public space used for everything from children's birthday parties to all-day concerts ending with fireworks.
She takes out her phone and plays music recordings that are blaring so loudly that the glass in her windows vibrates.
Her home is supposed to be a haven from the stress of work, but the noise "brings frustration, it makes me want to cry."
She has already been hospitalized twice for chest pains and is completely convinced that the noise is causing her stress, which is damaging her health.
"I feel a physical change, the noise definitely does something to my body," she says.
According to research by Dr Maria Foraster, who analysed evidence on noise for the World Health Organisation (WHO), traffic noise in Barcelona causes around 300 heart attacks and 30 deaths each year.
Across Europe, noise is linked to 12.000 premature deaths per year, as well as millions of cases of serious sleep disturbance and severe anxiety due to noise, which can negatively impact mental health.
I meet Dr. Maria Foraster in a café separated by a park from one of the busiest streets in Barcelona.
My sound meter shows that the noise from distant traffic here is above 60 decibels.
We can talk normally without having to raise our voices, but that's already an unhealthy level of noise.
The key number for heart health is 53 decibels, Dr. Foraster tells me, and the higher the noise level, the greater the health risks.
"A level of 53 decibels means we need to be in a fairly quiet environment," explains Dr. Foraster.
And that's only true during the day, and for a good night's sleep we need even lower noise levels.
"We need silence at night," says Dr. Foraster.
However, it's not just about the volume, as the way sound disrupts our peace and the degree of control we have over it affect our emotional reaction to noise.
Dr Foraster claims that the impact of noise on health is "on par with air pollution", but much harder to understand.
"We are used to understanding how chemical compounds can affect health and are toxic, but it is not so simple to understand that a physical factor, such as noise, affects our health much more than hearing," she explains.
A loud party can be a moment of joy that makes life worth living, but for some it is unbearable noise.
Traffic noise has the greatest impact on health because people's exposure to it is high.
But traffic is also the sound of going to work, shopping, or taking children to school.
Solving the noise problem involves asking people to change their lifestyles, which in itself creates new problems.
Dr. Natalie Miller from the Barcelona Institute for Global Health takes me on a walk through the city center.
We start from a busy street where my noise level meter shows more than 80 decibels, and then head to a quiet tree-lined avenue where the noise drops to 50 decibels.
But this street is different.
It was once a busy thoroughfare, but it has been transformed into a pedestrian area full of cafes and gardens.
From the shape of the flower beds, I can guess what the intersection once looked like.
Vehicles can still pass through here, but more slowly.
You remember that during an experiment in the lab we discovered that some sounds can calm the body.
"It's not completely quiet here, but the experience of sounds and noise is different," says Dr. Miller.
The initial plan was to create more than 500 so-called "superblocks", pedestrian-only zones created by connecting several city blocks into a single whole.
Dr. Miller conducted research that predicted a five to 10 percent reduction in city noise would prevent around "150 premature deaths" annually caused solely by noise.
And that, he says, would be "just the tip of the iceberg" in terms of health benefits.
However, only six superblocks were built.
City officials declined to comment.
Urbanization
Noise hazards continue to grow.
Urbanization is bringing more and more people into noisy cities.
Dhaka, the capital of Bangladesh, is one of the fastest growing megacities in the world.
This has also increased the number of vehicles on the streets, creating a cacophonous, continuous noise of sirens.
Artist Momina Raman Royal has been nicknamed the "lone hero" because his silent protests have drawn attention to the city's noise problem.
Every day, for about ten minutes, he stands at the intersection of several busy streets holding a large yellow banner, accusing drivers who honk too loudly of creating a serious problem.
He began this mission after the birth of his daughter.
"I want to stop all the siren noise, not just in Dhaka, but in all of Bangladesh," he says.
"If you look at birds, trees or rivers, nothing makes noise except people, so people are responsible."
But here too, the first signs of political action are visible.
Sjeda Rizwana Hasan, an environmental advisor and minister in the Bangladeshi government, told me she was "very concerned" about the health effects of noise.
The government is trying to reduce the use of sirens by strictly enforcing existing laws and through a campaign to raise awareness about the harmful effects of noise.
"This is impossible to solve in a year or two, but I think it is possible to make the city less noisy, and when people see that they feel better when the noise level is lower, I am sure their habits will change too."
Noise solutions can be difficult, complex and challenging to implement.
What I learned from this experience is to respect the spaces in our lives where we can simply escape the noise.
Because, as Dr. Masrur Abdul Qadeer from Bangladesh University of Professionals says, noise is "a silent killer and a slow-acting poison."
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Bonus video:
