What exempts a Catholic from going to Mass on Sunday at church? My grandparents are devout Catholics in the Midwest. They are fairly involved in their local church, however if they are away they will go to any close church. This much is fine, and I admire them for their faith and dedication. 

As they were born and raised in small Midwest towns during the 1940s, I believe that their dedication to this has to do with their interpretation of "remember the sabbath day" to mean that they need a church and priest.

Where I have concerns is that, last Sunday, the roads were icy enough that they were having trouble safely driving and walking. This is boosted by both of them reaching critical memory issues, and my grandmother not eating enough and relying on a walker. As such, I am concerned for their safety, and want to give them an alternative if the roads are too dangerous for them.

Since they are devout, I am looking for something in Catholic doctrine that defines whether or not a priest or physical church is necessary for worship. A preferred or additional verse would include some prioritization of your own health.

EDIT:
Thank you for the answers! I didn't realize that Mass was a compulsory idea since I come from a Protestant background. I'll see if my grandparent's friends aren't willing to pick them up. God Bless!
The first precept of the Church is to


  go to Mass and refrain from servile work on Sundays and holy days.


Grave inconvenience can exempt one from observing a precept of the Church. Grave inconvenience is, according to Fr. Hardon, S.J.,


  a sufficient reason to excuse a person from fulfilling certain positive precepts of the Church, not of themselves binding by the natural or revealed law. Also sufficient reason for delaying or even not performing certain actions, otherwise obligatory, provided there is a sincere desire to do so. 


However, the 3rd Commandment (Ex. 20:8-10),


  Remember that thou keep holy the sabbath day. Six days shalt thou labour, and do all thy works; but on the seventh day is the sabbath of the Lord thy God; thou shalt do no work on it


is "binding by the natural or revealed law".

So, even if one cannot without grave inconvenience observe the first precept of the Church, one still must sanctify Sunday, such as by extra spiritual reading, prayer, or even watching a live "internet Mass" or television or radio broadcast of a Mass, which can have spiritual benefits according to Father Connell Answers Moral Questions (Washington: Catholic University of America 1959) 75–6:


  One may participate in the benefits of the Mass without being actually present — namely, by directing one’s intention and devotion to the sacred rite. By hearing Mass over the radio one can certainly foster his devotion, and thus profit considerably from the offering of the Holy Sacrifice. Indeed, it could happen that one who participates in the Holy Sacrifice in this manner will gain much more benefit than many of those who are actually present.