expressions and paste

I have written a function to plot data which will be used for various different chemistries.
A simplified version is:
plot_data <- function(risk,levels,chem,sd2,measure){
  plot(risk, levels,main=paste ("per", sd2, measure, "\n in usual", chem))
}
The problem is with the title.
This works fine if the variable "chem" is just text, but if it is an expression then obviously it won't work.
I have experimented with various things and I am at a complete loss for how to insert an expression into the middle of 
a title.
If the expression was going in directly I would use main=expression(paste("text ", expression, " text")) but again this 
doesn't work.
Can anyone help?
thanks
Emily.