Monkeypox is continuing to spread across Europe and North America, and now British health officials are warning there could be some level of risk associated with plane travel.

In a document published Friday, the U.K. Health Security Agency (UKHSA) outlined the level of risk associated with different types of exposure to the virus.

The highest risk of contracting the virus comes via direct contact with an infected person’s skin, mouth, nose or bodily fluids, or contaminated clothes or bedding, the UKHSA said.

People in Britain deemed to be “high risk” contacts of confirmed monkeypox cases after being exposed to the virus in any of these ways are now being advised to self-isolate for three weeks.

Meanwhile, being seated directly next to a monkeypox case on a plane, sharing a car or taxi, or being within 1 meter of an infectious person without wearing appropriate PPE, were scenarios that would put an individual at "medium risk" of exposure to the virus, the UKHSA said.

Full PPE against monkeypox consists of an FFP3 mask, a long-sleeved gown, gloves and eye protection, according to the organization.

People found to have had any of these “medium risk” forms of contact with an infected person are not being advised to self-isolate by the UKHSA, but the health body said it would offer them a smallpox vaccine, ideally within four days of exposure to the virus.

Smallpox, which was eradicated from circulation decades ago thanks to vaccines, is from the same virus family as monkeypox, and so smallpox vaccines also provide protection against monkeypox.

While sitting directly next to an infected person on a plane fell into the UKHSA’s medium risk category, sitting within three rows of a monkeypox patient on a flight was categorized as a low-risk activity.

Someone who had this level of contact with an infectious person would not require a vaccine, the UKHSA said, and could carry on as normal after the flight as long as they remained asymptomatic.

People who were sat more than three rows away from a monkeypox case on the case were not medically considered to have had any contact at all with the patient.

Rare for public surfaces to become contaminated

Last week, the CDC reportedly confirmed that it was monitoring six individuals who sat within three rows of a British person who later tested positive for the virus.

David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, told Fortune on Monday that in most cases, monkeypox infections were caused by physical contact.

“That means if one person who is not infected touches a person’s lesion, or sore, who is infected, if that sore is open and weeping it can put virus on the skin that enters the other person’s body through a microscopic entry point,” he said in a phone call.

Heymann stressed that it would be very rare for surfaces in public spaces to become contaminated with the virus.

“It’s a disease that hasn’t been studied well, so we’re just beginning to understand it. But I think it would be wrong to say it’s everywhere, and it certainly isn’t transmitted by the respiratory means that COVID-19 is transmitted,” he explained.

“We know how it’s transmitted, and if we do a risk assessment ourselves before we have close physical contact, we should be able to prevent ourselves from becoming infected.”

This story was originally featured on Fortune.com