How do I let my boss know that I cannot take on another task as I am maxed out already?

Boss just asked me to join him and other auditors for a week long audit in September in addition to my other scheduled audits. I am already maxed out with my schedule this year. I do not want to get near the end of the year and have to ask assistance with my assigned audits. I need to send him an email soon about this. How do I explain my concerns?