74
Trace Elements in Abiotic and Biotic Environments
when grown in coastal regions and semiarid climatic zones. Some experiments have
shown that above half of Cl contents in radish and lettuce are absorbed by aboveground parts from evaporates of a soil arable layer (Kashparov et al. 2005).
Plants require relatively low levels of Cl for metabolic processes. It is concentrated in chloroplasts; however, its function in the photosynthesis is not fully
known (Marschner 2005). Chlorine stimulates activity of some enzymes, such as
cytochromeoxidase and phosphorylase, and plays a role in the osmotic system of
plants. Because Cl is supplied to plants from various sources, more concern is
about its toxicity than deficiency. However, Cl may be deficient to some plants
grown in soils with Cl contents below 2  mg/kg. Especially some crops (e.g.,
tobacco) require Cl level in soils within the range 10–15 mg/kg. The threshold Cl
content in soils is estimated at 2700 mg/kg. Higher Cl contents, especially in soil
solution, may be toxic to some plants. The less tolerant plants (e.g., beans and apple
trees) are inhibited by Cl concentrations from 460 to 673 mg/L, whereas resistant
plants (e.g., tobacco, cereals, cotton, beets, and spinach) can grow at Cl contents
within 887–3546 mg/L (Kabata-Pendias 2011). HCl and Cl 2 , gases emitted from
industrial sources, are the most toxic to plants. Very high Cl contents up to 3300
and 3550 mg/kg in tree leaves and vegetable and fruits, respectively, are affected
by industrial pollution (Nersyan 2007). Sometimes, trees may secrete excess Cl,
mainly with the falling of leaves in autumn.
Chlorine contents in crop plants are variable (in mg/kg) and are as follows: cereal
plants, 10–20; sugar beet leaves, 100–200; potato tubers, 1300–5500.
11.6 HUMANS
Chlorine is present in the body of humans as chloride ions. On an average, an adult
human body contains approximately 95 g of Cl, 900 mg/kg in the bone, average of
0.03% in the blood, and 0.2%–0.5% in the tissue (Emsley 2011).
The most harmful route of exposure is from breathing Cl gas. Exposure may also
result from skin or eye contact with Cl gas or by swallowing Cl-contaminated food
or water.
When Cl gas enters the body, as a result of breathing, swallowing, or skin contact, it reacts with water to produce acids. The acids are corrosive and damage the
cells in the body. Most harmful Cl exposures are from its inhalation. Health effects
typically begin within seconds to minutes. The health effects of Cl gas in humans
come from accounts of soldiers exposed during gas attacks in World War I. Other Cl
exposures are at work; accidental exposures occur due to leaks or explosions of storage tanks or due to the mishandling of bleach solutions or swimming pool chemicals.
Effects of exposure to low Cl concentrations may be limited to irritation of the eyes,
upper respiratory tract, and skin, whereas exposure to high concentrations may cause
serious pulmonary effects and death. There is limited information on neurological
effects in humans exposed to Cl gas. Oral exposure is not a relevant route of exposure to Cl gas in humans or animals (ATSDR 2010a).
Chlorine is used to clean swimming pool water to prevent contamination and
bacterial overgrowth. It is not the safest method, but is probably the most common.
It is confirmed that long-term exposure to chlorinated pools can cause symptoms