General Science

CHEMISTRY: ENVIRONMENTAL CHEMISTRY

Overview

• Environmental chemistry is the study of chemical and biochemical phenomena that occur in natural places
• Environmental chemistry is used to detect and identify the nature and source of pollutants, including
  • Heavy metal contamination of land by industry. These can transported to water bodies and taken up ingested by living organisms
  • Nutrients leaching from agricultural land into water sources
  • Urban pollutants runoff. Typical pollutants include petrol and other fuel, metals, nutrients and sediments
• Common environmental phenomena arising out of contamination include acid rain, soil salination and ocean acidification

WATER QUALITY PARAMETERS

• Dissolved Oxygen (Oxygen Saturation)
  • It is a relative measure of the amount of oxygen dissolved in water. 
    
  • Supersaturation (excess of oxygen) can be harmful to organisms and also cause decompression sickness
  • It is expressed in mg/l
    
• Chemical Oxygen Demand (COD)
  • COD is used to indirectly measure the amount of organic compounds in water
    
  • It is expressed in mg/l, which indicates the amount of oxygen consumed per litre of water
    
• Biochemical Oxygen Demand (BOD)
  • BOD measures the rate of uptake of oxygen by microorganisms in water
    
  • BOD is measured at a temperature of 20 C and over a period of 5 days in the dark
    
  • BOD is widely used to determine the threshold at which treated wastewater can be re-introduced into the environment
  • Pristine rivers have a BOD of below 1 mg/l. Municipal sewage treated effectively by a three-stage process would have BOD of 20 mg/l or less
• Total Dissolved Solids (TDS)
  • TDS is a measure of combined content of all inorganic and organic substances contained in a liquid
    
  • TDS is generally not considered a primary pollutant, but is used to indicate the aesthetic characteristics of drinking water
  • High TDS levels generally indicate hard water
  • Drinking water is expected to have a TDS of 100 mg/l or less
  • TDS is different from TSS (Total Suspended Solids). The former are those solids that are small enough to pass through a filter of size 2 um, while the latter are those solids that cannot pass through
    

ENVIRONMENTAL POLLUTION PHENOMENA

Acid rain

• Acid rain is form of rain that is unusually acidic i.e. has low pH
• Acid rain is mostly caused by emission of sulphur, nitrogen and carbon which react with water molecules in the atmosphere to produce acids
• The biggest human activity causes of acid rain include coal-based power plants, factories and automobile emissions
• It can also be caused by natural phenomena such as
  • lightning strikes (which splits nitrogen compounds)
  • volcanic eruptions (which release large quantities of sulphur dioxide)
• Natural (unpolluted) rain is slightly acidic with pH of 5.2 due to the reaction of carbon dioxide with water to produce carbonic acid
• Acid rain has many adverse effects including
  • Damage to aquatic animals
  • Damage to soil chemistry by killing off essential microbes
  • Loss of forests and vegetation
  • Human illnesses such as cancer, asthma and other diseases
  • Damage to buildings and historical monuments (esp. those made of limestone and marble)

Ocean acidification

• Ocean acidification is the continuing phenomenon of decreasing pH in the world’s oceans
• Between 1751 and 1994, ocean pH is estimated to have decreased from 8.179 to 8.104 (decrease of 0.075). Ocean pH is expected to decrease by a further 0.3-0.5 by 2100
• This acidification is mainly the result of uptake of carbon dioxide from the atmosphere. The world’s oceans naturally absorb carbon dioxide from the atmosphere, indirectly mitigating climate change
  
• Ocean acidification adversely affects marine organisms especially calcifying organisms like corals, crustaceans and molluscs, and also affects other organisms by entering the food chain

Soli salination

• Salt affected soils are caused by excess accumulation of salts at the soil surface
• Salt can be transported to the soil surface by capillary action from salt-laden water tables, or by human activity
• Increasing soil salinity adversely affects soil quality and vegetation
• Human activities that increase soil salinity include
  • Land clearing
  • Aquaculture activities (shrimp farms etc)
  • Irrigation (over a period time causes deposition of salts)
• The adverse effects of salination include
  • loss of soil fertility
  • damage to infrastructure (such as roads etc)
  • damage to plant growth and yield
  • deterioration of underground water quality
  • soil erosion

COMMON ENVIRONMENTAL TOXINS

1. Chlorofluorocarbons
   1. They are organic compounds that contain carbon, chlorine and fluorine
   2. Examples of CFCs include Freon, Teflon
   3. CFCs have been widely used as refrigerants, propellants (in aerosols) and solvents
   4. The use of CFCs has been banned under the Montreal Protocol due to their adverse effect on the ozone layer
2. Endocrine disruptors
   1. Endocrine disruptors are substances that affect the function of natural hormones in the body
   2. Food is the main source of exposure to endocrine disruptors
   3. There are five main types of endocrine disruptors:
      1. DDT
      2. Polychlorinated biphenyls
      3. Bisphenol A
      4. Polybrominated diphenyl ethers
      5. Pthalates
3. DDT
   1. Dichlorodiphenyltrichloroethane (DDT) is one of the most well-known synthetic pesticides
   2. DDT is one of the most effective and simple to deploy pesticides, especially to fight mosquitoes that cause malaria and typhus
      
   3. DDT has significant adverse effect on aquatic life, insects and humans (esp. diabetes and reproductive disorders)
      
   4. It is a significant reproductive toxicant for certain bird species, and is a major reason for the decline of the bald eagle, brown pelican peregrine falcon and osprey. This is the main reason DDT use has been banned
      
   5. The use of DDT for agricultural use has been banned under the Stockholm Convention, however it can still be used for disease vector control (mosquito eradication)
      
4. Polychlorinated biphenyls (PCBs)
   1. PCBs are a class of industrial compounds
   2. They are used mainly as industrial coolants and lubricants
   3. Exposure to PCBs increases the risk of skin cancer, brain cancer and liver cancer. Additionally it also increases childhood obesity and the risk of developing diabetes
      
   4. The use of PCBs was banned in 1977
5. Bisphenol A (BPA)
   1. BPA is an organic compound with two functional phenol groups
   2. BPA is used as a building block of several important plastics and plastic additives
   3. It is found commonly in water bottles, plastic food containers and the lining of infant formula cans
   4. The use of BPA has been linked to diabetes, mammary and prostrate cancers, reproductive problems, obesity and neurological disorders
   5. BPA use has not been banned
6. Polybrominated diphenyl ethers (PBDE)
   1. PBDEs are a class of compounds used as flame retardants
   2. They are used commonly in televisions, computers, electronics, carpets, bedding, clothing car components etc
   3. PBDEs have the potential to affect thyroid balance, and contribute to a variety of neurological and developmental disordersincluding learning disabilities and low intelligence
   4. Many of the most common PBDEs were banned by the European Union in 2006
7. Phthalates
   1. Phthalates are esters of phthalic acid
   2. They are mainly used as plasticisers to soften polyvinyl chloride (PVC)
   3. Phthalates are found in soft toys, flooring, medical equipment, cosmetics and air fresheners
   4. Phthalates have been shown to have adverse effects on the male reproductive system
   5. The EU and the US have begun phasing out widespread use of phthalates
8. Dioxins
   1. Polychlorinated dibenzodioxins (PCDDs) are a group of polyhalogenatated compounds
   2. The main sources of Dioxins include
      1. By-products in the manufacture of organochlorides
      2. in the incineration of chlorine containing substances (like PVC)
      3. bleaching of paper
      4. natural sources like volcanoes and forest fires
   3. Dioxins accumulate and build up in the food chain (bioaccumulation)
   4. Health effects of dioxins include
      1. Severe form of acne called chloracne
      2. Abnormalities in teeth enamel of children
      3. Nervous system pathology
      4. Thyroid disorders
      5. Diabetes
      6. Damage to immune system
   5. Exposure to dioxins has been shown to affect the ratio of male to female births, such that more females are born than males
      

BIOLOGY: VACCINES

Overview

• A vaccine is a biological preparation that improves immunity to a particular disease
• Vaccines were first used by Edward Jenner (England) in the 1770s to inoculate against small pox using the cow pox microbe
• Vaccines have resulted in the eradication of small pox, one of the most contagious and deadly diseases known to man
• Other diseases like polio, measles, mumps, typhoid etc are have been significantly reduced. Currently, polio is prevalent in only four countries: Afghanistan, Pakistan, Nigeria and India
  Mechanism of action
• A vaccine is usually made from a weakened or dead form of the microbe that it is intended to fight
• It stimulates the body’s immune system to recognise the microbe as foreign, and destroy it and remember it
• When the same microbe re-appears later, the immune system easily recognises and destroys it
• When the body recognises the virulent microbe attack, it
  • Neutralises the target microbe before it can enter body cells
  • Destroys infected cells before the microbe can spread to other cells and multiply

Types of vaccines

• Killed vaccines: these are vaccines that contain micro-organisms that have been killed using chemicals or heat. Eg: influenza, cholera, bubonic plague, polio, hepatitis A
  
• Attenuated vaccines: these contain live attenuated (numerous) micro-organisms. These are usually live viruses that have been cultivated under conditions which disable their virulent properties, or use closely-related by less dangerous micro-organisms.These vaccines provide more durable immune response and are preferred type for healthy adults. Eg: yellow fever, measles, rubella, mumps, typhoid
  
• Toxoid vaccines: inactivated toxic compounds that cause illness. Eg: tetanus, diphtheria 
  
• Subunit vaccines: these use protein subunits instead of the entire micro-organism as a vaccine. Eg: Hepatitis B vaccine (which uses only surface proteins), Human Papilloma Virus (HPV) vaccine (which uses subunits of influenza virus)
  

Effectiveness of vaccines

• Vaccines do not guarantee complete protection from a disease
• This could be due to
  • Host’s immune system may not respond adequately
  • Host may have lowered immunity (such as due to diabetes, HIV, steroid use etc)
  • Host may not have a B cell capable of producing antibodies to that particular antigen
• The efficacy of a vaccine depends on a number of factors
  • The disease itself
  • The strain of vaccine
  • Following the schedule of vaccinations
  • Individual host factors
  • Genetic and ethnic predisposition
• Most vaccines use adjuvants to boost immune system response. Adjuvants are compounds added to the vaccine that increase the immune response, without having any specific antigenic effect by themselves. 
  
• Aluminum salts like aluminium phosphate and aluminium hydroxide are the most common adjuvants used
  
  

List of important vaccines

Vaccine	Disease	Type	Notes
Anthrax vaccine	Anthrax	Protein subunit
Bacillus Calmette-Guerin (BCG)	Tuberculosis	Live bacteria
DTP	Diphtheria Pertussis (whoopoing cough) Tetanus
Gardasil (Human Papilloma Virus (HPV))	Cervical cancer	Protein subunit
Polio vaccine	Polio	Killed/inactivated	Polio is prevalent only in humans Currently polio has been eradicated from all countries except Afghanistan, Pakistan, Nigeria and India
MMR	Measles Mumps Rubella
Meningococcal vaccine	Meningococcus
Rabies vaccine	Rabies	Attenuated
Yellow fever vaccine	Yellow fever	Attenuated