Table 1.

Representative biomarkers reflecting the human status (* bpm: beat per minute, ** bpm: breaths per minute)

Biomarker type	Biomarkers	Reflecting human status (Note)	Measurand/concentration range (Note)
Biomarkers in body fluid	Na+	Hydration state[ 244 ]	>291 mmol h−1 (in sweat)[ 244 ]
	K+	Hydration state[ 245 ]	>200 mg h−1 (in sweat)[ 245 ]
	Lactate	Fatigue level[ 246 ]	<25 × 10−3 m (in sweat)[ 195 ] <17.3 × 10−3 m (in blood), < 1.6 × 10−3 m (in saliva)[ 247 ]
	Cortisol	Psychological stress[ 248 ]	8.16–141.7 ng mL−1 (in sweat)[ 249 ]
	Dopamine	Psychological stress[ 250 ]	0.01 × 10−6 –1 × 10−6 m (in extracellular fluid)[ 251 ]
	Urea	Metabolism status[ 252 ]	100–250 mmol L−1 (in sweat)[ 253 ]
	Histamine	Acute stress[ 254 ]	>65 ng mL−1 (in blood)[ 255 ]
	Pyrrole	Psychological stress[ 255 , 256 ]	>20 µg dL−1 (in urine)[ 255 ]
	Neuropeptide Y	Psychological stress[ 257 ] Thermal stress[ 258 ]	>129 pm (in cerebral plasma)[ 259 ]
Vapor‐phase biomarkers	Acetone	Metabolic rate[ 260 ]	10–76 ppm (in exhaled breath)[ 261 ]
	Isoprene	Hypoxia/respiratory status[ 262 ]	274.9–474 ppb (in exhaled breath)[ 263 , 264 ]
	Ethane	Psychological stress[ 265 ]	5–9 ppb (in exhaled breath)[ 266 ]
	Malondialdehyde	Cellular oxidation status[ 267 ]	30–130 nmol L−1 (in exhaled breath)[ 267 ] 78.2–107.7 nmol L−1 (in exhaled breath)[ 268 ]
	Isopropyl alcohol	Toxic health (aerotoxic syndrome)[ 32 ]	50–250 ppb[ 269 ]
Noninvasive physical biomarkers on skin	Skin temperature	Thermal stress (by mean temperatures)[ 37 ] Psychological stress (peripheral temperature)[ 270 ]	25–36 °C (on peripheral spot on skin)[ 100 ]
	Skin impedance/conductance	Psychological stress (on palm and volar wrist)[ 41 ] Hydration state[ 174 ]	2–20 µS[ 271 ]
	Skin hardness	Thermal stress[ 47 ]	20–30 duro (shore 00)[ 47 ]
	Pulse wave (heart rate)	Physical and psychological stress[ 41 , 272 ]	50–200 bpm* (0.83–3.66 Hz)[ 108 ]
	Sweat rate	Thermal stress[ 55 ]	0–110 g m−2 h (from no stress to strong heat stress)[ 273 ]
	Breathing rate	Psychological state[ 274 ]	4–65 bpm**[ 275 ]
	Blood pressure	Physical and psychological stress[ 276 ]	$\frac{>120\text{mm}\text{Hg}(\mathrm{systolic})}{>80\text{mm}\text{Hg}(\mathrm{diastolic})}$ [ 50 ]