You aren’t the only one who’s been drinking more.

During the first week of sheltering in place, Bay Area residents drank more alcohol than usual — 42% more than usual, in fact, according to data from BACtrack, a San Francisco company that produces smartphone-connected breathalyzer devices.

The idea for the study came from people’s anecdotal impressions that drinking was up, said BACtrack CEO Keith Nothacker. “We said, let’s go check the data. Sure enough, drinking had exploded post-shelter-in-place.”

The BACtrack findings dovetail with reports of increased demand for alcohol delivery: San Francisco online retailer Wine.com reports that its spirits sales have surged 400% since shelter-in-place orders started, for example. Newly relaxed regulations from California’s Department of Alcoholic Beverage Control have made it legal for bars and restaurants to deliver cocktails now, too.

To produce the data set, BACtrack compared users’ blood alcohol concentration levels during the first week that shelter-in-place orders went into effect for six Bay Area counties, beginning March 17, with the two-week period preceding it. The data looked at users of BACtrack devices who live in those counties, which totaled around 500 breathalyzer tests during that time period, Nothacker said — but he added that BACtrack collects “millions” of data points worldwide, and already are seeing similar results from other metropolitan areas.

Who is taking breathalyzer tests at home, you might wonder? Many of BACtrack’s breathalyzer products are intended for personal use — for instance, for people who want to test their level of inebriation before they drive. (The company also produces some devices intended for professional use, including for law enforcement.) The personal devices are small and portable and can deliver results to an app on your smartphone. BACtrack does not automatically collect user data, Nothacker said: The company captures information only from those who elect to have their breathalyzer results saved to the cloud, and that data is then anonymized.

According to its findings, the average blood alcohol concentration in the Bay Area — at least among users who opted to share their results — on Tuesday, March 17 was 0.045%, which represents a 125% jump from the pre-quarantine Tuesday average of 0.02%. The fact that March 17 was also St. Patrick’s Day could explain the excess drinking — or maybe it was just how people decided to cope with the first full day of sheltering in place. On Sunday, March 22, the average BAC was 0.077%, a 133% climb over the average pre-quarantine Sunday levels of 0.033%, and rather close to the legal limit for driving.

The only day in which Bay Area BAC levels dropped was Friday, March 20, when the average BAC was 0.02%, a 50% decrease from the pre-quarantine Friday level of 0.04%.

“This data exactly matched our thesis going in,” Nothacker said. “Drinking was up on previously non-drinking days — Sunday, Monday, Tuesday — whereas Friday was down dramatically.” Why? “People aren’t going to happy hours on Fridays. They’re not going to bars or restaurants.”

When the six Bay Area counties announced their shelter-in-place orders, they were the strictest of their kind in the country. It’s little surprise, then, that between March 17 and March 23, BAC levels in the Bay Area (at an average 0.047% for that seven-day period) outpaced those in California (0.033%) and the U.S. (0.036%).

But since March 17, many other regions have instated similar social-distancing measures. And as BACtrack has begun to look into those numbers, familiar patterns have emerged. “Right now we’re in the process of crunching New York data, and we’re seeing similar results,” Nothacker said. “Nationwide we’re seeing the same thing: higher-than-average BACs and the same day-of-week shift.”

BAC levels don’t tell us the number of drinks people are having, but instead track the concentration of alcohol in a person’s blood stream. If two people drink exactly the same amount of alcohol, they may not feel the same level of inebriation, since factors including biological sex, weight, medications and eating can all influence how alcohol is metabolized and absorbed by the body.

According to the Centers for Disease Control and Prevention, a BAC level of 0.02% can often result in “relaxation” and “slight body warmth,” a level of 0.05% in “lowered alertness” and “release of inhibition” and a level of 0.08% — the threshold at which it is no longer legal to drive a car — in compromised muscle coordination, among other symptoms.

Theoretically, people are driving less during shelter-in-place, which eliminates the risk of drunk driving. That would also mean that people aren’t using breathalyzer devices to see if they’re safe to drive as much right now. But Nothacker suspected that there is another reason people are using their breathalyzer devices more: boredom.

“People are at home, they’re watching a lot of Netflix,” he said.

The silver lining? “We think that it’s an amazing opportunity for people to test themselves safely at home and understand what one drink is and how that impacts their BAC,” Nothacker said. Ideally, when bars re-open and dinner parties at friends’ homes are again deemed safe, breathalyzing now could help people make smarter decisions in the future.

Esther Mobley is The San Francisco Chronicle’s wine critic. Email: emobley@sfchronicle.com Twitter: @Esther_mobley Instagram: @esthermob