Half-life (t1/2, DT50):

Time required for the concentration of a reactant in a given reaction to reach a value that is the arithmetic mean of its initial and final (equilibrium) values. For a reactant that is entirely consumed, it is the time taken for the reactant concentration to fall to one-half of its initial value. [Source: IUPAC]

Hazard:

Inherent property of a substance, agent or situation having the potential to cause undesirable consequences (e.g. properties that can cause adverse effects or damage to health, the environment or property). [Source: International Code of Conduct]

Hazard assessment:

A process designed to determine the possible adverse effects of an agent or situation to which an organism, system, or (sub)population could be exposed. The process includes hazard identification and hazard characterization. The process focuses on the hazard, in contrast to risk assessment, where exposure assessment is a distinct additional step. [Source: IPCS/OECD]

Hazard characterization:

The qualitative and, wherever possible, quantitative description of the inherent property of an agent or situation having the potential to cause adverse effects. This should, where possible, include a dose–response assessment and its attendant uncertainties. Hazard characterization is the second stage in the process of hazard assessment and the second of four steps in risk assessment. [Source: IPCS/OECD]

Hazard identification:

The identification of the type and nature of adverse effects that an agent has an inherent capacity to cause in an organism, system, or (sub)population. Hazard identification is the first stage in hazard assessment and the first of four steps in risk assessment. [Source: IPCS/OECD]

Hazard quotient (HQ):

Ratio of toxicant exposure (estimated or measured) to a reference value regarded as corresponding to a threshold of toxicity. Note: If the hazard quotient exceeds unity, the toxicant may produce an adverse effect, but normally this will require a hazard quotient of several times unity; a hazard quotient of less than 1.0 indicates that no adverse effects are likely over a lifetime of exposure. [Source: IUPAC]

Health-based exposure limit:

Maximum concentration or intensity of exposure that can be tolerated without significant effect (based on only scientific and not economic evidence concerning exposure levels and associated health effects). [Source: IUPAC]

Highest residue (HR):

Highest residue level (expressed as mg/kg) in a composite sample of the edible portion of a food commodity when a pesticide has been used according to maximum GAP conditions. The HR is estimated as the highest of the residue values (one from each trial) from supervised trials conducted according to maximum GAP conditions, and includes residue components defined by the JMPR for estimation of dietary intake. [Source: JMPR]

Highly Hazardous Pesticides:

Pesticides that are acknowledged to present particularly high levels of acute or chronic hazards to health or environment according to internationally accepted classification systems such as WHO or GHS or their listing in relevant binding international agreements or conventions. In addition, pesticides that appear to cause severe or irreversible harm to health or the environment under conditions of use in a country may be considered to be and treated as highly hazardous. [Source: International Code of Conduct]