除草劑的作用主要是干擾植物的正常生長活性，大部分藥劑僅針對植物所特有的生理或生化過程，如光合作用、胺基酸生合成等目標位置的阻斷，因此對動物的毒性較殺蟲劑或殺菌劑為低，甚至低於許多日常消費品。但這也不意味著除草劑在任何狀況下都是絕對安全的，如非選擇性除草劑巴拉刈具皮膚刺激性，高急毒性；因無解毒劑，誤飲或吞服後極易致命。除草劑毒性作用視藥劑的種類、暴露時間的長短與劑量的高低。毒性程度也隨動物的品種、年齡、性別、營養狀況與曝露途徑而改變，曝露途徑包括口服、呼吸及皮膚。皮膚和眼睛也會受到化學物質引起之刺激。慢毒性則包括致突變性，致畸胎性及致癌性。雖然大部分除草劑之口服急毒性都落在低毒或極低毒範圍內，但大面積噴施、高劑量接觸或易於揮發等劑型，還需考量到皮膚及呼吸系統毒性，也影響到施用後重入施用區的間隔時間。除草劑施用後釋放到環境中，部分被植物吸收或淋溶到根分布的區域，達到控制雜草的效果，部分則轉移到非目標施用區，不但造成藥劑的浪費，雜草防治率的降低，還可能引起非目標生物的危害及土壤與地下水的污染。藥劑在環境中的行徑，除了植物的吸收外，還包括吸附、揮發、飄散、徑流、淋溶、吸收和分解等轉移過程，使除草劑遠離目標位置發生不同程度的移動情形。農藥在水、土中之殘留量主要受化學結構特性、水土理化性質及降解因素所支配，使用普遍及分解慢之藥劑可對大環境造成污染。此外，土壤殘效期過長，也可能引起後作或間作栽植之敏感作物發生藥害。因此選擇適合的雜草防除藥劑時，除考慮藥效與藥害因素外，除草劑的毒性及環境安全性也應列入考量。

Many modern herbicides kill weeds selectively by interrupting metabolic processesthat are unique to plant life. For this reason, their systemic toxicities in mammals arevery low, which is due to the biochemical pathways influenced by these chemicals donot exist in mammals, such as photosynthesis, amino acid or chlorophyll biosynthesis.However, some herbicides have a poison risk if handled carelessly, and many areirritating to eyes, skin, and mucous membranes. Paraquat, for example, is highly toxicorally and can be fatal. Herbicides have the mode of entry into the human body same asother pesticides, through the skin (dermal), by swallowing (oral) and by breathing(inhalation). Therefore, it is important that all herbicides need to be handled carefullyand in a manner consistent with their labeling. Herbicides that fall directly upon the soilor are washed onto the soil can undergo a number of processes including degradation andtransport. Degradation processes include biological degradation by edaphicmicro-organisms, chemical and photochemical transformations. Transport of herbicideswithin the soil can occur downward into the soil profile (leaching), across the soil surface(runoff), or upward into the air (volatilization). Most herbicides are organic compoundsand are therefore basically unstable in the environment. Inherent instability is essential toprevent these materials from accumulating in the environment as compounds arerepeatedly used. Accumulation of pesticides not only poses environmental hazards, butprohibits rotation to sensitive crops. Therefore to have a complete overview of the fateand environmental toxicity, biological and chemical approaches have to be combined.