2. Assessment of quality of evidence (GRADE).

Comparison	N Studies	1. RoB?	2. Inconsistent?	3. Indirect?	4. Imprecise?	5. Pub bias?	6. Large ES?	7. DR?	8. Opp Conf	Qualitya
Outcome noise
Legislation vs no legislation	1 ITS	yes	1 study	no	no	1 study	yes	no	no	very low (1)
One HPD vs another HPD	1 RCT 4 CBA	2 yes	no	no	no	not shown	no	no	no	low (1)
HPD+Instruction vs HPD‐instruction	2 RCT	2 no	no	no	yes	not shown	na	na	na	moderate (4)
Information vs no information	1 RCT (2 arms)	1 yes	1 study	no	yes	1 study	na	na	na	low (1, 4)
Outcome hearing loss
One HPD vs another HPD (TTS)	2 CBA									no data
Muffs vs plugs	2 CBA	2 yes	no	no	yes	not shown	no	no	no	very low (1,4)
Frequent HPD vs less frequent use	1 CBA	1 yes	1 study	no	yes	1 study	no	no	no	very low (1)
HLPP vs audiometry	1 RCT	1 yes	1 study	no	no	1 study	na	na	na	moderate (1)
HLPP+exposure information vs HLPP‐information	1 CBA	1 yes	1 study	no	yes	1 study	no	no	no	very low (1,4)
Frequent HPD in HLPP vs less	5 CBA	5 yes	no	no	yes	not shown	no	no	no	very low (1,4)
HLPP vs no exposure	7 CBA	7 yes	no	no	yes	not shown	no	no	no	very low (1,4)
Follow‐up vs no follow‐up	1 CBA	1 yes	1 study	no	yes	1 study	no	no	no	very low (1,4)
HLPP+long shifts vs HLPP normal	1 CBA	1 yes	1 study	no	yes	1 study	no	no	no	very low (1,4)

1‐5 Reasons for downgrading: 1. Risk of bias/Limitations in study design 2. Inconsistency between studies. 3. Indirectness of PICO 4. Imprecision of the results 5. Publication bias. 6‐8 Reasons for upgrading: 6. Large effect size. 7. Dose‐repsonse relationship 8. Confounding opposes the direction of the effect;
 na= not applicable; 1 study = only one study available and impossible to assess consistency or publication bias

^aFinal grading of quality of evidence, between brackets domain that led to down/upgrading the quality.