Mental health experts are worried: Depression is on the rise in the US, especially in teenagers. From 2005 to 2015, depression rates in kids between the ages of 12 to 17 spiked, and the suicide rate for teenage girls is now the highest it's been in four decades.

Experts aren't sure what's causing the disturbing trend, but they're concerned about the mental health of the country.

In New York City, coaches are working to recruit and train a kind of civilian mental-health army of 250,000 volunteers.

The hope is that people trained in "mental health first aid" will be better able to start conversations, lend a helping hand, and share compassion for friends, colleagues, neighbors and other fellow New Yorkers who are dealing with mental health issues.

New York residents who volunteer get a free, day-long training - I recently participated in one.

My fellow volunteers and I were taught some simple ways to spot the difference between someone experiencing a bad mood or a few bad days and a person with more serious, long-term depression.

The coaches suggested a four-word approach to checking in on friends and loved ones

If you're worried that you or someone you care about may be suffering from depression, but you're not quite sure how serious it is, ask yourself how the person is doing when it comes to four key happiness measurements.

Is you/your loved one's mental state having an impact on the person's ability to:

  • Live
  • Laugh
  • Learn
  • Love

If these four pillars of life are feeling compromised by a persistent cloud of sorrow or indifference, it may be a sign that depression is at hand.

It's pretty likely that you know someone dealing with depression

The World Health Organization (WHO) estimates that around 5.9 percent of the US population is dealing with a diagnosable depressive disorder. That means more than 1 in every 20 Americans are suffering from depression at any given time.

Depression is the single largest contributor to "non-fatal health loss" worldwide, according to the WHO. It affects around 4.4 percent of the globe, though those rates vary from place to place.

Some of the highest depression rates in the world are among women in Africa, while some of the lowest are in men in the Western Pacific islands, the WHO says.

When health care professionals in the US diagnose depression, they use a manual called the DSM-5, or "Diagnostic and Statistical Manual of Mental Disorders."

It lists several common signs of depression, and defines depression as a condition that lasts for more than two weeks and impacts a person's ability to go about their life, enjoying the activities that typically make them happy.

Depression might change how a person thinks and feels about the world, and can uproot how they would otherwise go about an average day.

In some cases, it can make it impossible for a person to get out of bed in the morning. And it can have an impact on other daily routines, like how a person sleeps, eats, and works.

Only a trained mental health professional can officially diagnose depression, but when it comes to on-the-fly "first aid" for depression, these tell-tale signs can be a first warning that it might be good to ask for help or see a counselor.

If you're worried about depression, here are some additional resources:

In New York, you can contact health care professionals at this link. They're available by text message or online chat there as well.

If you're having suicidal thoughts and are in the US, call the National Suicide Prevention Lifeline at 1-800-273-8255.

There's a number of resources available for your country here.

There are also new apps coming online that aim to provide some of the benefits of therapy on the go. These include Woebot, a texting-based system, but they're untested and by no means a substitute for a mental health professional.

The National Institute of Mental Health also offers a full list of resources for depression on its website.

This article was originally published by Business Insider.

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