The Council on the Safe Transport of Hazardous Articles (COSTHA) recently shared an interpretation from Mexican transportation authorities that makes it clear
that Mexico expects hazmat labels to be placed with the "vertex upwards," I.e., as a square-on-point, or a square at a 45-degree angle, as required by the UN Model Regulations 5.2.2.2.1.2. Mexico requires the diamond orientation even if the package is too small to fit a 100 mm x 100 mm hazmat label.
Mexico’s interpretation differs from US DOT’s in that the HMR
does not prohibit shippers to place hazmat labels in a square orientation when placing a label “correctly” is not possible
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Mexico Less Lenient With Labels
Mexico’s dangerous goods
Normas Oficiales Mexicanas (NOMs) lays out the nation’s requirements for hazmat shippers and carriers. COSTHA recently requested clarification of
NOM-003-SCT/2008 (in Spanish), which prescribes standards for hazardous materials labels.
Point 8.1.2 of the NOM states “Las etiquetas tendrán Ia forma de un cuadrado. colocado con un vértice hacia arriba,” which translates to “The labels will have the shape of a square, placed with the vertex up.”
COSTHA asked whether Mexican authorities would allow a hazmat label to be placed in a square orientation on packages too small to accommodate the diamond orientation
Mexican transport authorities responded as follows: “For the above, answering your question I inform you that based on the basics of the identification of products considered dangerous, it is necessary that the pictograms are displayed in the form of a diamond, since the emergency personnel is familiar with this identification and may act if appropriate and necessary, for this reason you cannot place the labels in square orientation.”
(Translation from the original Spanish, available here.)
Alternative Hazmat Labeling Options for Small Packages
On packages too small to fit a hazmat label in a diamond orientation, the UN Model Regulations allow shippers to use a reduced-size label so that the label may be placed in accordance with the rules. PHMSA also allows a reduced size labels on small packages
(49 CFR 172.407(c)(1)(i)). 
PHMSA offers additional options for labeling too-small hazmat packages at
49 CFR 172.406(b), such as affixing the label to a securely affixed tag or affixing it by other suitable means. These alternative labeling methods are allowed for packages that
do not contain radioactive materials (Class 7) and are smaller than the 3.9-inch-by-3.9-inch (100 mm-by-100 mm) minimum label size. Shippers may also use alternative labeling means on packages with “irregular surface to which label cannot be satisfactorily affixed.”
Given Mexico’s strict interpretation of the hazmat labeling requirements, your best bet may be to invest in reduced-sized labels or larger packagings if you ship dangerous goods south of the border. IATA issued guidance to
relax its DG label standards earlier this year. Hazardous Materials Shipper Training (49 CFR, IATA, IMDG)
Meet DOT and IATA training mandates in cities nationwide in 2018. Be confident your shipments are in full compliance with the latest 49 CFR and DGR requirements for ground or air transport, earn CM Points and CEUs, and leave with trusted resources to simplify compliance and support your decisions.