US coronavirus cases are climbing to a 3rd peak. Color-coded charts show how risky everyday activities are.

• Coronavirus infections are in the US are growing towards a third peak.
• Experts fear this could be the largest surge yet as Americans start spending more time indoors and pandemic fatigue sets in.
• The color-coded charts below show how to evaluate the riskiness of various activities.

The US reported more than 70,400 new COVID-19 cases on Friday — the highest daily total since July.

Twenty-one states have set single-day records for new infections in the past 10 days.

Indeed, COVID-19 cases are surging for the third time in the US since the start of the pandemic — a spike experts think could become the largest yet. It's happening, they say, because more people are spending time indoors as the weather gets cold, and Americans are simultaneously feeling fatigued by pandemic safety measures.

"If the rates never get that low, and basic public-health measures are not universally adopted, and then you bring people indoors to share a meal together, you're kind of putting together the perfect storm," Ingrid Katz, an assistant professor at Harvard Medical School, previously told Business Insider. "Unfortunately this was completely anticipated."

But research continues to show that some activities and types of gatherings are far riskier than others. The four color-coded charts below reveal which types of excursions are safest right now and which are most dangerous.

Four factors that increase your risk

Your risk of a coronavirus infection primarily has to do with how close you get to people and for how long, a June study showed. Four factors raise your chances of catching the virus: spending time in enclosed spaces, being among crowds, circumstances in which social distancing is though, and prolonged close contact with others (defined as being within 6 feet of an infected person for more than 15 minutes total during a 24-hour period starting two days before that person feels sick).

Here's how activities stack up in terms of coronavirus risk, based on those four factors:

Levels of mask-wearing and talking matter, too

There are two issues with the CDC's 6-foot rule, however. It's based on science that's nearly 80 years old, and it doesn't take into account how well ventilated a place is, meaning how easily air can circulate. Studies show that poorly ventilated spaces could foster coronavirus transmission.

So many experts caution against assuming that a distance of 6 feet keeps us 100% safe all the time. Instead, they say, it's useful to consider certain aspects of an activity, like how well a space is ventilated and whether people are wearing masks. How much talking, shouting, or singing is going on also matters, since studies have shown that talking loudly and forcefully exhaling can lead a person to spew viral particles farther than 6 feet.

In August, researchers from Oxford University and the Massachusetts Institute of Technology released a color-coded system that ranks coronavirus transmission risk based on ventilation, mask-wearing, and occupancy level.

The researchers concluded that activities in low-occupancy spaces were less risky across the board, even in situations with poor ventilation.

How to decrease your risk

If you decide to engage in an activity on the higher end of the risk spectrum, like working out in a gym, there are still ways to lower your risk.

Good handwashing, especially before eating or touching your face, as well as keeping a safe distance from others snd using face masks can all help.

"It appears that people are wearing masks and socially distancing more frequently as infections increase, then after a while as infections drop, people let their guard down and stop taking these measures to protect themselves and others — which, of course, leads to more infections. And the potentially deadly cycle starts over again," Dr. Christopher Murray, director of the University of Washington's Institute for Health Metrics and Evaluation, said in a statement.

The institute works to predict the coronavirus' future spread; its most recent model anticipates that at least 170,000 more Americans may die from COVID-19 by February 1. The current US coronavirus death toll sits at more than 221,000.

"We're seeing a roller coaster in the United States," Murray added.

Aria Bendix contributed reporting.