They're two different chemicals with different physical properties (like melting point). The nitrate (extra oxygen, white solid, easily dissolves in water) is naturally occurring and converts to the nitrite (white to yellowish solid, easily dissolves and holds water). The nitrite is an important industrial antimicrobial agent. In air, water-loving sodium nitrite will form sodium nitrate slowly. Acute overdosing on the nitrate is harder. The nitrite, much easier.
Both are linked to both short-term and long-term serious health effects, but most research in humans is around the in-the-body conversion of nitrate to nitrite. Healthy gut environments (pH, microbes) typically help convert low concentrations of nitrite to nitrate without acute problems (excretion). Methemoglobinemia (iron oxidation in hemoglobin meaning the oxygen carrying structure binds far less oxygen ) is the acute toxic effect of nitrite poisoning (from ingestion or inhalation) and shows up as low blood oxygenation with cardiac abnormality (dysrhythmias and blood circulation failure) leading ultimately to worsening central nervous system function. The CNS failure shows up, progressively, as sleepiness, dizziness, coma, and/or convulsions.
A quick note. The primary toxic effects of nitrite poisoning can be reversed if a subject is found... by the administration of MetHb reductase (enzyme that works with two abundant blood chemicals {NADH/NADPH} to reduce/restore the iron positive ion at the center of methemoglobin).
