In suburban Maryland, a high school biology teacher sneaked into the bedroom of a 17-year-old German exchange student and tried to get her to perform a sexual act. In California, a teacher had sex with a 15-year-old female foreign-exchange student living in his home.

Though believed to be rare, those incidents of abuse of foreign-exchange students last year and other cases prompted the US State Department to propose new rules to protect the youths. Adults in host families would have to run their names through their state's sex-offender registry, according to the rules proposed earlier this month. Volunteers who work with the nearly 30,000 foreign-exchange students who come to the United States annually would have to submit to criminal background checks. The government will take public comment on the rules until Oct. 11, then later enact them.

But host families in Massachusetts, as well as leaders of foreign-exchange programs, say the cost, and the potential intrusiveness of the measures may not be worth it. Background checks would not have picked up first-time offenders such as those in the Maryland and California cases, they note.

The Maryland teacher pleaded guilty to assault charges stemming from the December incident. The California man, who also pleaded guilty, was sentenced last year to three years in prison.

Some say extensive screening could hurt attempts to recruit more host families.

''It's a panacea approach: If everyone fills out the paperwork, then all the bad people will show up in the paperwork," said Nathan Felde of Newburyport, who, along with his wife, has hosted foreign students. ''And it seems to me that if someone has bad intentions, they're the least likely to show up in the paperwork."

Foreign-exchange students are among a school's most vulnerable students because they are unfamiliar with American laws and customs, and may feel shy about speaking out. The proposed rules bring youth-exchange programs in line with what school systems, youth groups, and nonprofits have done for years.

Programs currently do not have to run criminal checks of host families. Most use references and visit families' homes to determine whether they should serve. One of the nation's largest agencies that places students, the Cambridge-based EF Foundation for Foreign Study, doesn't conduct criminal background checks, unless something in a family's application or interview raises red flags.

''It will create a little bit more effort on our part if it's implemented. But we certainly don't want to not examine it just because it would be more work," said Megan Allen, director of government and school relations for EF Foundation. ''Our program and our students are too important for us to dismiss it on those grounds."

No agency collects annual statistics on how many foreign-exchange students have been victims of crimes, abuse or otherwise. Under the proposed rules, allegations of sexual misconduct must be reported to the State Department.

In the past 10 years, when there has been no such requirement, the department recorded five reports of alleged sexual abuse, out of 250,000 students who visited. However, those backing tougher restrictions said the actual number of cases is far higher because many victims do not report the allegations.

''We have had a lot of interest in this from concerned citizens," State Department officials said in a statement. ''We have examined the situation fully and decided it was necessary to build in an extra level of protection for our youth-exchange participants."

Because foreign-exchange students are scattered all over the country and placed through as many as 100 different programs, what happens to them is difficult to track. Massachusetts had 368 foreign-exchange students last year, according to the Council on Standards for International Educational Travel, an industry group.

John O. Hishmeh, the council's executive director, said the proposals are a sign of the times, rather than a reflection of a large problem among exchange programs.

''I don't think anybody is going to fight making students safer and prevent what's preventable," Hishmeh said. ''The question is, what type of checks and how long it takes and what are the costs involved?"

Officers of youth-exchange programs plan to meet in Chicago next month to discuss the proposed rules. Many have unanswered questions, Hishmeh said. For example, if a background check reveals that a program volunteer had a drunken-driving arrest 20 years ago, would that prevent him or her from serving? And is it effective if crimes committed in one state do not show up in another state's records?

But others say the State Department didn't go far enough. In Oceanside, Calif., families who have hosted exchange students in the past established the Committee for Safety of Foreign Exchange Students to push for tighter restrictions.

Sally Arguilez Smith, a spokeswoman for the committee, said host families should undergo criminal background checks because those checks cover more than just sexual crimes.

Smith criticized youth-exchange programs for citing costs as a barrier, saying firms get thousands of dollars in government grants and charge students between $5,000 and almost $9,000. The US Department of Justice is working to put all sex-offender registries online, so they can be searched for free. Criminal background checks, such as the one run by Massachusetts' Criminal History Systems Board, can cost between $15 and $30 each.

''I would have them face a child that's been raped and ask them why they didn't want to" conduct a background check, Smith said. ''If you're serious about taking in a child, you should have no problem saying, 'I have nothing to hide in my background, and I allow this check.' "

Anand Vaishnav can be reached at vaishnav@globe.com.