H-acid (or 1-napthol-8-amino-3,6 disulfonic acid) is an azo dye (an alcohol + an ether) *which exists as a white crystal or grey powder. Large quantities of sulphuric and nitric acids, common salt,-iron filings, caustic soda, lime and napthalene are needed for its manufacture, which can cause extensive damage to standing crops and the long term fertility of the soil by changing its pH and salt concentrations. The nutrient uptake of the roots are affected, and often the delicate root hairs which absorb nutrients are totally destroyed. Napthalene is a known mutagen and also adversely affects plant roots and soil microflora.

H-acid is highly toxic and carcinogenic in nature. Large quantities of gypsum sludge ( FeC03 and CaS04) and acidic effluents are generated during the process. The effluents are highly toxic and carcinogenic. The pollutants are very difficult to treat as many of them are refractory in nature. Strict environmental regulations in western countries forced the closure of the units there. Till 1978, the processed water from these units was collected and dumped into the North Sea. This is not allowed any more. Drinking water contaminated with H-acid wastes is said to cause liver damage.