It seems that every Bay Area transit rider has a story of hopping on BART or Caltrain only to be hit by a wave of unpleasant smells. So with mask mandates on the outs, many passengers may be wondering: If BART can’t filter out the mystery scent from a few seats away, how can the system keep me safe from an unmasked passenger emitting the COVID-19 virus?

There is some good news, experts say. Your BART car might smell, but the air filtration systems that reduce COVID-19 transmission are stronger than what’s typically at work in your home, office or the restaurant where you just had lunch.

BART and other transit agencies have been busy tweeting about their optimal airflow in the days since a federal court ruling in Florida led airlines, airports and transit agencies across the country to drop a federal mandate and make COVID-19 masks optional. On Monday, BART shared a tweet of one passenger measuring low carbon dioxide levels, which are a proxy for fresh air.

“We need everyone to understand that our air is replaced every 70 seconds,” BART said in a tweet earlier this month.

So why the stench?

“Smell molecules are generally gases, and gas molecules are a billion times smaller relative toa regular COVID-19 respiratory molecule,” said Alex Huffman, an aerosol scientist at the University of Denver. “If you want to eliminate smell, you’ll have to use a charcoal filter — an entirely different system.”

That means the fumes emanating from a day-old burrito — less than one-1,000th of a micron in size — are passing right through BART’s air filters, while most COVID-19 virus particles attach themselves to respiratory droplets that can be up to 100 microns and are getting trapped, Hoffman said.

Many transit systems upgraded their filtration systems during the pandemic.

Filters are rated on a 1-to-20 scale called Minimum Efficiency Reporting Value, or MERV. BART is using a MERV-14 filter, which is the highest among major Bay Area transit operators and is often used in hospital settings. Caltrain uses a MERV 13 filter. AC Transit and VTA buses both use MERV 7 filtration systems, which are less effective at trapping aerosol droplets but still provide more protection than filters in many indoor spaces.

There’s another air quality factor to consider: how often the air is replaced. Across all major public transit, the air is replaced with new or filtered air about 50 times an hour — that’s far higher than in your home, office or a restaurant. The high rate of air replacement should also tame any smells coming from inside the bus or train cabin.

“Six air exchanges per hour is considered good. In our homes, we have less than one per hour,” said Jorge Luis Salinas, an infectious disease expert at Stanford University. “One of the ways the virus is transmitted is by accumulating in a cloud inside the train. What BART has implemented prevents that possibility.”

But even with generally lauded air ventilation systems on Bay Area public transit, public health experts say well-fitting N95 masks are the best way to prevent contracting the disease or spreading it to others, especially on crowded trains and buses.

“Even if the rate of ventilation is high, if you’re standing right next to someone and they’re breathing in your face, then you’ll get a high dose,” said Huffman. “Wearing a mask reduces the near-field big puff of aerosols that would go into your face.”

Despite BART’s efforts to improve airflow, the agency will consider at its board meeting Thursday reinstating its mask mandate. Other transit operators are also facing pushback from advocates and riders calling on the agencies to impose independent mask policies as the Biden administration appeals last week’s federal court ruling striking down the mask requirement that covered planes and public transit.

In the meantime, many riders are resorting to mental mathematics to decide if they want to enter a train or bus and sit alongside an unmasked passenger.

“If you had 100 people on a BART car, you would probably have on average about two people with COVID,” said Dr. John Swartzberg, an expert in infectious disease at UC Berkeley. Swartzberg, who also is not dining indoors, said he will not take on the added risk of riding BART due to personal health concerns.

“We’re still in the midst of a pandemic, and we still have a significant amount of virus circulating,” he said. “The fulcrum point is your own risk tolerance.”

Last week, rider Teresa Lewis faced this new dilemma on her daily BART commute into San Francisco. She has a son at home with disabilities and would rather not put him at risk of catching COVID-19.

“This morning there were just lots of people coughing without their masks,” said Lewis, who decided to keep riding BART with a mask as the majority of people continue to do so too. If that changes, though, she’ll drive instead. “For me, it’s a no-brainer.”