Hazard classes and the SDS for storage decisions

Before you can decide where a chemical belongs, you need to know what it is and how it behaves. The supplier has already done most of that work: the hazard classes on the label and the information in the safety data sheet (SDS) tell you what the product can do and how the supplier recommends storing it. HSE-102 taught you to read labels and SDSs in general. This lesson focuses on the parts that drive storage decisions, and on how to turn a shelf full of products into a short list of storage groups.

Physical, health and environmental hazards

The Globally Harmonized System sorts hazards into three families. Physical hazards describe what a chemical can do to property and people through fire, explosion or reaction: flammable liquids, gases, aerosols and solids, oxidizers, self-reactive substances, organic peroxides, pyrophorics, self-heating substances, substances that emit flammable gases in contact with water, gases under pressure, explosives and corrosives to metals. Health hazards describe what a chemical can do to the body: acute toxicity, skin corrosion or irritation, serious eye damage, sensitization, germ cell mutagenicity, carcinogenicity, reproductive toxicity and organ toxicity. Environmental hazards describe harm to aquatic life and the ozone layer.

For storage, physical hazards matter most, because they decide what a chemical must be kept away from and what kind of storage space it needs. Health hazards still matter: highly toxic chemicals may need locked storage and extra containment, and corrosives can attack containers, shelving and skin. Environmental hazards matter for spill prevention and waste.

The SDS sections that matter for storage

An SDS has 16 sections in a fixed order. Four of them are essential for storage decisions, and three more are useful.

Section 2, hazard identification, lists the hazard classes and categories, signal word, hazard statements and pictograms. This tells you the storage groups the product belongs to. Section 7, handling and storage, gives the supplier's specific storage advice: temperature limits, incompatible materials, container types, ventilation and precautions such as keeping the container tightly closed or storing under nitrogen. Section 9, physical and chemical properties, gives the flash point, boiling point, vapour pressure, vapour density, pH and other data used to classify flammable liquids and to judge how readily vapour builds up. Section 10, stability and reactivity, lists conditions to avoid, incompatible materials and hazardous decomposition products. This is the main source for compatibility decisions.

Section 5 (firefighting measures) tells you whether water can be used on the product, which matters for where it sits relative to sprinklers. Section 6 (accidental release measures) tells you what absorbent and neutralizer a spill kit needs. Section 8 (exposure controls and personal protection) tells you which glove material and eye protection to stock near the storage area.

Reading section 10 carefully

Section 10 is where many storage mistakes start, because suppliers often write it in general terms. Phrases such as strong oxidizing agents, strong acids, strong bases and reducing agents describe families of chemicals, not specific products. To use them, you need to know which of your other products fall into those families. Concentrated nitric acid, hydrogen peroxide above about 8 %, calcium hypochlorite, sodium hypochlorite solutions, nitrates, permanganates, chlorates and perchlorates are oxidizers. Hydrochloric, sulfuric, phosphoric and acetic acids are acids. Sodium hydroxide, potassium hydroxide, ammonia solutions and many alkaline cleaners are bases. Metal hydrides, sodium metal and some organometallic reagents are strong reducing agents that also react with water.

If section 10 says incompatible with water or reacts violently with water, the product needs a dry location away from sprinkler heads, sinks and water lines, and the fire department needs to know it is there. If it says avoid heat or store below a stated temperature, it needs temperature control and should not sit near heaters, in direct sunlight or in an unheated shed that bakes in summer.

From hazard classes to storage groups

Most workplaces do not need a separate storage location for every hazard class. They need a small number of storage groups whose members are compatible with each other. A common scheme for small users uses eight to ten groups:

Some products belong to two groups. Glacial acetic acid is both flammable (category 3) and corrosive. Ammonium nitrate is an oxidizer and may be an explosive in some forms. For these, follow the most hazardous property and keep them away from the incompatibles of both groups. Lesson 4 shows how to handle these cases with a compatibility table.

# Assign products to storage groups from their GHS classes (simplified rules).
# Order matters: the first rule that matches wins, so the most hazardous comes first.
RULES = [
    ("water-reactive", lambda c: "water-reactive" in c or "pyrophoric" in c),
    ("oxidizers",      lambda c: "oxidizer" in c and "acid" not in c),
    ("oxidizing acids",lambda c: "oxidizer" in c and "acid" in c),
    ("flammables",     lambda c: "flammable" in c),
    ("acids",          lambda c: "acid" in c),
    ("bases",          lambda c: "base" in c),
    ("toxics",         lambda c: "acute tox 1-3" in c),
    ("general",        lambda c: True),
]
products = {
    "acetone": {"flammable"},
    "nitric acid 70%": {"oxidizer", "acid", "corrosive"},
    "pool chlorine": {"oxidizer", "corrosive"},
    "glacial acetic acid": {"flammable", "acid", "corrosive"},
    "sodium hydroxide": {"base", "corrosive"},
    "sodium metal": {"water-reactive"},
    "sodium cyanide": {"acute tox 1-3"},
    "sodium chloride": set(),
}
for name, classes in products.items():
    group = next(g for g, test in RULES if test(classes))
    print(f"{name:20s} -> {group}")

acetone              -> flammables
nitric acid 70%      -> oxidizing acids
pool chlorine        -> oxidizers
glacial acetic acid  -> flammables
sodium hydroxide     -> bases
sodium metal         -> water-reactive
sodium cyanide       -> toxics
sodium chloride      -> general

The simplified rules show the logic. Glacial acetic acid goes to the flammables group because flammability is checked before acidity; in practice it is stored in a flammable cabinet with its own corrosion-resistant tray, away from oxidizers and bases. Nitric acid goes to its own oxidizing acids group because it is both an acid and a strong oxidizer that reacts dangerously with organic acids and solvents.

Labels as a storage tool

Labels do more than meet the hazard communication rules. They are the main way anyone can tell, at a glance, where a container belongs. Many organizations add a coloured storage-group dot or band to each container when it is received. Some chemical suppliers use their own colour schemes for storage groups; if you adopt one, use it consistently and explain it on a posted chart, because colour schemes differ between suppliers and between organizations. Never rely on colour alone: the written storage group and hazard pictograms should also be visible.

Two dates also belong on every container: the date received and the date opened. These support first-in, first-out use, shelf-life tracking and peroxide-former testing (lesson 7). A permanent marker and a few seconds at the receiving bench are all it takes.

When the SDS is old or missing

Storage decisions are only as good as the SDS they are based on. An SDS more than a few years old may use an earlier classification, may not reflect a reformulated product, or may come from a different supplier than the product on the shelf. Suppliers must provide a current SDS with hazardous products, and workplaces must keep one available for every hazardous product they use. If an SDS is missing or clearly outdated, ask the supplier for the current version before deciding where to store the product, and keep the product in a conservative location, such as with the most restrictive group it might belong to, until you have it. Never guess a storage group from the product name alone; trade names such as 'Green Clean' or 'Citrus Degreaser' say nothing reliable about flash point or reactivity.