Principles for characterizing the potential human health effects from exposure to nanomaterials: elements of a screening strategy

Table 1 Recommendations for measuring exposure during inhalation studies

Metric	Measurement Recommendation
Mass – off-line	E (coupled with on-line)
Mass – on-line	E
Size distribution – off line	E
Size distribution – on line	E/D
Surface area – off line	O
Surface area – on line	O
Number – off line	N
Number – on line	E

E: These measurements are considered to be essential.
D: These measurements are considered to provide valuable information, but are not recommended as essential due to constraints associated with complexity, cost and availability.
O: These measurements are considered to provide valuable but non-essential exposure information.
N: These measurements are not considered to be of significant value to inhalation studies.