40 CFR § 27.34 - Evidence.
(a) The presiding officer shall determine the admissibility of evidence.
(b) Except as provided in this part, the presiding officer shall not be bound by the Federal Rules of Evidence. However, the presiding officer may apply the Federal Rules of Evidence when appropriate, e.g., to exclude unreliable evidence.
(c) The presiding officer shall exclude irrelevant and immaterial evidence.
(d) Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or by considerations of undue delay or needless presentation of cumulative evidence.
(e) Although relevant, evidence may be excluded if it is privileged under Federal law.
(f) Evidence concerning offers of compromise or settlement shall be inadmissible to the extent provided in Rule 408 of the Federal Rules of Evidence.
(g) The presiding officer shall permit the parties to introduce rebuttal witnesses and evidence.