Like everyone else watching the Boston Marathon videos, I was awed by the courage and skill of the first responders. But I was equally awed by the ordinary bystanders who stepped up in those terrifying moments.

The low-focused bomb blast produced horrific wounds to people’s lower extremities. Good Samaritans fashioned primitive but workable tourniquets to staunch the bleeding and save lives. Random passers-by intervened to comfort wounded and frightened people. The truth is that everyday people are often the real first responders in public health emergencies, be they large and small.

Watching the footage, I couldn’t help wondering: If I were present in some emergency, would I actually know how to help?

Few of us will ever face mass casualty incidents as we saw in Boston, not to mention larger catastrophes such as Katrina or 9/11. But many of us will be the first person on the scene when a child slices her hand on a broken glass, when a diabetic co-worker passes out, when a parent suffers a stroke or heart attack. Our individual and collective competence in addressing such emergencies can be the difference between life and death.

But most of us know far too little about what to actually do when confronted with a stranger (or a relative, or a friend) who needs immediate help.

It’s been awhile since I earned my Boy Scout first aid merit badge.  Like millions of others, I’ve let my own skills atrophy. So I contacted Don Lauritzen of the American Red Cross.

Much of what he told me was pretty basic. The Red Cross promotes the concept of check, call, and care. Encountering an emergency, one should check the scene for safety and check any person who needs help. Someone should then call 911 and provide care until help arrives.

Lauritzen described core first aid skills everyone should know: How to control bleeding, how to care for burns, how to respond to seizures or fainting. At a more advanced level, one should know how to respond to cardiac and breathing emergencies. That includes knowing Cardiopulmonary resuscitation (CPR), knowing how to operate an Automated External Defibrillator (AED), and how to provide first aid for choking.

The public health literature doesn’t provide much data regarding Americans’ basic proficiency in first aid. That information gap is, itself, revealing. A society committed to population health would make such skills a priority, and would pursue evidence-based strategies to impart these skills in schools, workplaces, and other venues.

A small but revealing literature on out-of-hospital cardiac arrests does provide useful clues. The Cardiac Arrest Registry to Enhance Survival (CARES)  provides some of the best data here. If you collapse in the street with a cardiac arrest, correctly-performed CPR by no means works miracles, but it definitely increases your chance of survival. Yet less than half of witnessed cardiac arrests include bystander-initiated CPR. There are also clear disparities in CPR receipt between non-Hispanic whites and other groups. Cities also vary in the usual depressing ways correlated with socioeconomic status. Residents of Austin who suffered cardiac arrests in public had a 55 percent chance of some bystander administering CPR. The comparable figure in Houston was 39 percent.

We need to take care of each other. At the highest level of social policy, this web of mutual obligation provides a basic argument for social insurance. At the level of everyday life, these same obligations take more elemental forms.  We should immunize ourselves and our kids.  We should sign our organ donor cards. We should learn what to do if a sibling falls into a diabetic emergency or if a bicyclist goes down and suffers a head injury.

The American Red Cross offers first aid courses in classrooms and on the Web. It offers pretty impressive smartphone apps, which include instructions and videos on topics from allergies to seizures and stroke. These apps can call 911 while providing useful instruction.

Lauritzen told me that roughly four million people take these courses every year. That’s a good start. But it’s not nearly enough in a nation of 313 million people. If you are watching TV footage of Boston’s tragedy and wondering how you could help, here’s one thing to do: Sign up for one of these courses.

Harold Pollack is Helen Ross professor at the School of Social Service Administration at the University of Chicago. He is also co-director of the University of Chicago Crime Lab and an executive committee member of the Center for Health Administration Studies (CHAS) at the University of Chicago. He has published widely on the connections between poverty policy and public health, and is a nonresident fellow at the Century Foundation.