As reported in our February 5, 2020, blog item, the American Conference of Governmental Industrial Hygienists (ACGIH®) Threshold Limit Values for Chemical Substances (TLV®-CS) Committee included carbon nanotubes on its 2020 list of chemical substances and other issues under study. Being placed on the under study list indicated that the TLV®-CS Committee had selected carbon nanotubes for development of a threshold limit value (TLV®). ACGIH® has now released its two-tier under study list. Tier 1 lists the chemical substances and physical agents that may move forward as a notice of intended change (NIC) or notice of intent to establish (NIE) in the upcoming year, based on their status in the development process. Tier 2 consists of those chemical substances and physical agents that will not move forward, but will either remain on or be removed from the under study list for the next year. Carbon nanotubes are included in Tier 2. If carbon nanotubes are included on the 2021 under study list, stakeholders will have an opportunity to submit substantive data and comments.