About 4 million difeerent chemicals are in use today and about 10% more are produced each year, out of which about 300 different come into commercial use. Daily, about 0.1 million chemicals are used; 20,000 of these are being reviewed by USEPA under RCRA and TSCA (by NTP). These include a44 priority pollutants as well as hazardous and toxic chemicals (like pesticides, heavy metals). 80,000 different organic chemicals (excluding petrochemical products) are produced in large amounts; many of these are without any adverse effects, some are produced or contain harmful byproducts, while others are directly harmful to health &/or ecosystem. The concern is if these come in human use or produced in large amounts or reach the environment or are directly ecotoxic. In USA these chemicals are regulated by EPA, FDA, and OSHA. In Europe OECD and internationally FAO and WHO regulate their commercial use.

Chemicals are toxic because they can exert toxic effects, which may be irreversible. The irreversible damage to a vital organ/s can eventually lead to mortality. The following tables provide some information to realize that chemicals that we and other organisms are exposed to, by design or by accidents, can vary a great deal in their toxic manifestations. Therefore, one has to be careful when comparing the amount of environmental chemicals. The toxicity of these chemicals x amounts can give us a better estimate of their harmful effects.

RELATIVE TOXICITY OF CHEMICALS

TABLE 1. Relative toxicity of various common chemicals.
___________________________________________________________________________________
                                                TOXICITY                                            RATING
AGENT                          LD₁₀₀       LD₅₀          amount to kill
                                              mg/Kg
_____________________________________________________________________________________

sugar                           >15,000                              a lot                           NONTOXIC

alcohol                       --15,000    10,000               gallon                         SLIGTLY TOXIC
salt                             --15,000      4,000
morphine sulfate             5,000           900               1 pt                           MODERATELY TOXIC
phenobarbital, sodium
                                       5,000         150

DDT                              3,000          300                  ounce                        VERY TOXIC
nicotine                             500          100              tea spoon                      EXTREMELY TOXIC
d-tubocurarine                50                   0.10           a taste                        SUPERTOXIC
TCDD, 2,3,7,8-              5                    0.001
Botulinus toxin               <5                    0.00001
_____________________________________________________________________________________
 

TABLE 2. Relative toxicity of air pollutants
___________________________________________________________________________________
                                             TOLERANCE LEVEL*                  RELATIVE TOXICITY
POLLUTANT                          ppm             ug / m                          (wighting factor)
____________________________________________________________________________________

CO                                           32               40,000                                    1
HC                                                               19,300                                    2
SO                                             0.50             1,430                                    28
NO                                             0.25               514                                    78
Particulate                                                          375                                  107
__________________________________________________________________________________
* at 25⁰ C, 760 mm Hg   1 ppm = (M.W x 10 / 24,470) x ug /m³
                                                         ug / m³= ppm x ( M.W x 10 / 24,470)
 

The environmental chemicals can be classified on the following general basis:

1. PHYSICAL STATE ........dust, gas, fumes, vapors, odors, fog, mist, radiation, liquid, solvents
2. CHEMICAL NATURE.....irritant, corrossive, water-soluble, fatty
3. USAGE.............................pesticides, fertilizers, oil, detergents, metals, plastics
4. LABELLING
            for transportation.........expolsives, flammables, oxidants, corrossive
            for toxicity..................supertoxic, extremely toxic, moderately toxic
5. TOXIC ACTION............neurotoxic, teratogens, carcinogens, hepatotoxins, chemotherapeutics
6. EXPOSURE....................environmnetal, occupational, clinical, forensic
7. SITE OF ENTRY...........local toxicant, systemic toxicant
 

In environmental toxicology the effects of a chemical or chemical groups include both temporal and spatial concerns. For example if you are applying the herbicide 2,4-D to your lawn: you may be the local and immediate target if contaminated. The applied herbicide with time can affect insects and other animals on your lawn. As time passes the residues can reach neighbors via wind or water or humans and other animals (walking on lawn) and this process may extend beyond the neighbourhood with time affecting larger ecosystem and the process may continue untill global contamination occurs in several years (if it does). Inside our body the 2,4-D can be transported by blood to various organs. It can be depositted in certain specific organ for storage or can reach the sensitive organ that may be the target of its toxicity. Inside the cell of the target organ it can exert molecular effects which can affect the physiology and behavior and even the survival. These are also time- and space-dependent effects. While inside the organism the chemical can be biochemically altered (mostly by liver enzymes) and the metabolite/s may exert their toxic effect or may be less toxic and excretable in feces, urine, sweat, milk, etc.
 

ENVIRONMENTAL TOXICOLOGY OF CHEMICALS focuses on followings:
* CHEMICAL
* EXPOSURE
*TOXICOLOGY
* ORGANISMS
 * POPULATIONS
 

TOXIC EFFECT OF A CHEMICAL CAN BE:
 LOCAL:  burn e.g. acid
 SYSTEMIC  entry, storage, transport, metabolissm, excretion    sequence
Terminal effect : NS, CVS, Visceral organs, muscle, bone
 DURATION:  acute, chronic, immediate, delayed
 SEVERITY:  reversible, irreversible
 NATURE:   general, selective (Frog heart : 0.02ug of acetylcholine=.0l6% area)
 INTERACTION  synergism, antagonism, additive
 

FATE & EFFECT OF A CHEMICAL IN  ORGANISMS INCLUDES STUDIES OF:
Toxicity: LD₅₀, LC₅₀,EC₅₀
Toxicokinetics
Bioaccumulation, storage
Biotransformation
Biodisposition
Toxicodynamics
Molecular Toxicology
Risk extrapolation/assessment
 

EFFECTS OF A CHEMICAL ON POPULATIONS
ECOTOXICOLOGY:     AQUATIC: freshwater, marine
                                        TERRESTRIAL: wildlife, epidemiology,
                                        MICROBIAL
                                        STATISTIC MODELS
 

TOXICITY EVALUATION

1. EXPOSURE
    ROUTE/SITE OF ENTERY : intended use, environmental
   ORAL: drinking, food
   INHALATION  : lung
   TOPICAL     : skin, eye
   OTHER       : iv, ip, nose, eyes, etc.

2.  FREQUENCY
   SINGLE/ ACUTE    24hr or less           4 hr inhalation, repeated     (SHORT-TERM      1 wk + )
   REPEATED
    Subacute    2-4 wk.
    Subchronic  1-3 mo.
    Chronic     3 mo. (Life Time )

TOXICITY ASSAYS
* Use species closely resembling the target/man
    Man is 10-time more vulnerable
    All carcinogens in man are carcinogenic in an experimental mammals

*  LARGE DOSE allowed by EPA, FDA, and OECD
* WHOLE ANIMAL TOXICITY
 *ORGAN TOXICITY
    Reduces number of animals to be used
    The effect occurs more frequently & can be detected

*  PLAN TOXICITY BIOASSAYS & STUDIES
* ACUTE: dosage-response relationship
* SUBCHRONIC, CHRONIC, & LIFE-TIME EXPOSURE
*SPECIAL STUDIES
* From the animal data the risk to human populations can be calculated

*HUMAN RISK ASSESSMENT
Probability & severity of risk, cost/benefit
Public policy about the chemical
 

ACUTE TOXICITY BIOASSAY:
INTENDED USE : ROUTE & SPECIES (Rat, mice, dog)

  High dose for effect; MTD

•   Lethality
•  Route
•  both sexes  at least 10 animals/dose/ 14 days
•  food withheld the night before
•  Observe: morbidity, lethargy, behavior , intake, output, symptoms, weight, etc.

  DERMAL     rabbit skin  2g/kg, 24 hr, wipe

  INHALATION  4 hr.

  SKIN & EYE IRRITATION  rabbit    skin : shaved
      0.5 ml or 0.5 gm/in  /4 hr; control  covered under 4 gauge patches       erythema, eschar, edema, corrosion
      repeat

       Eye  : 0.1 ml or 0.l g instilled into one eye       examined at various intervals, control eye

      SENSITIZATION  skin, albino guinea pig
            Inject intradermally, 3 times weekly, alternate day for 3 wk. After the 10th injection wait for 2 wk.
            challenge by a final injection

SUBCHRONIC  TOXICITY BIOASSAY. 90 days, oral

     rat 15-20 of each sex; dog  4-6/sex
     MTD + 2 doses (0 %, 10 %, 100 %)

    Observe  body wt, intake, symptoms,  mortality, hematology, clinical chemistry, urinalysis
    gross microscopic exam. of tissues

 CHRONIC    Mice 1.5-2 yr, rat 2-2.5 yr(30/dose/sex), dog
   Special studies
   Lowest Observed
         Effect Level
         Adverse Effect Level
   No-Observed Effect Level  NOEL,NOAEL,NEL
   Subthreshold
   Highest dose without toxicity
   Radiation has no NEL
 
 

BODY weight temperature, food & water intake, water output, posture, behavior
RESPIRATION rate, dyspnea, gasping, apnea
SKIN  dry, itching
SEDATION  sleep, fear, confusion, dizziness
EYE  miosis, lacrimation, nystagmus"
NEUROMUSCULAR tremors, convulsions, ataxia, prosbutim, vomiting, diarrhea, dizziness, muscle tone
HEART rate, B.P., pulse, EKG
BLOOD cbc, hematocrit, electrolytes, osmolarity, sugar, ammonia, urea, bilirubin, HbO; AchE, SGOT,
                 SGPT, alkaline phosphatase, CPK, LDH
ALLERGIES hay fever, asthma, sinus headache
 

 TOXICITY BIOASSAY   at least l0 animals per dose; at least 3 doses,
        95% confidence limits
        Slope of dosae-reponse curve
        Threshold: onset of toxic response
        Response : nonlinear or linear
 

 TABLE 4. FIXED DOSE or FIXED TIME
_______________________________________________________
     % RESPONDING            S.D.(NED)              PROBIT
_______________________________________________________
              0.1                               -3                                  2
              2.3                               -2                                 3
             15.9                              -1                                4
             50.0                                0                                5
             84.1                               +1                               6
             97.7                               +2                               7
             99.9                               +3                               8
_______________________________________________________

 DOSE................................Solid:      mg/kg,  mg/m
 CONCENTRATION......... Water:    mg/kg, mg/L (ppm)
                                             Air:         ug/m³ (ppb),  umole/m³

 Log₁₀ dose is used for chemicals, while arithematic dose for: radiation effects

 ED-₅₀ ET_-50        LD-50/ ED-50=  TOXICITY/THERAPEUTIC  INDEX (l0 is used,slope)
 TD-₅₀ TT-₅₀        LD-1/  ED-99=  MARGIN of SAFETY
 LD-₅₀ LT-₅₀        1-dose LD-50/90-dose LD-1 = CHRONICITY INDEX; 1--no accumulation,  90--accumulation.
 

After an extensive analysis, Roberts concluded that most of the programs provided useful and comparable LC_, estimates. The exception to this was the UGPROBIT. The commercially available packages in SAS and SPSS had the advantages of graphical output and a method for dealing with control mortality. DULUTH-TOX
 

SAFETY FACTOR FOR HUMAN HEALTH (CANCER) RISK ASSESSMENT

 reduce NOEL by a safety factor

• if  human toxicity data is available:  NEL/10 for human variability

•  if no human data available then: animal NEL / 100  most sensitive animal species with 95%         upper confi.limit
•  if  no animal data: NEL/  l000
•   WHO   :  NEL/10-20,000 depending on the nature of the  data  (deficency, biochemistry)

• 1-5% responseis usually NEL
• 10% response = Benhmark response (BMR) and the dose is Benchmark dose (BMD)

Acceptable Daily Intake= Reference Dose (daily intake with NOAEL) = NOAEL / UF*MF or = BMDx / UF*MF
        (ADI)                                           (RfD)

Suppose
    epidemiological NOAEL = 50 mg / Kg / Day, then RfD = 50/10 = 5
    critical study (90-day rat) LAEL = 50 mg/Kd/Day, then RfD= 50 / 10 x 10 x 10 x 10 = 0.005
    These are used in Health Advisory
 

VIRTUALLY SAFE DOSE AND ACCEPTABLE DAILY INTAKE

VSD (=NEL / 100-5,000)
  The daily human dose/exposure that would result in a theoretical excess (over the background) risk of
            cancer 1 in a million, < 1 in 10 million is desirable.
  Toxicity tests are not designed for chemical's safety. VSD is an extrapolation of the dose-response curve.
  The 10^-6 risk  should not be underestimated.
        * USNAS: a lifetime exposure to 120 mg /day saccharin can produce 0.001 to 5,200 cases of bladder
                          cancer in one  million humans

ADI
       This is used by WHO for food and water intake of residues of pesticides and food additives, etc.
        This is the sum of residues in each food and drink x per capita food and water consumptions = permitted
         " the daily intake (mg/Kg body wt) of a chemical, which during an entire lifetime appears to be without
        appreciable risk  on the basis of all known factors at the time"

        Regulatory Agencies:  FDA, Codes Alimentaris Commission (120 countries) set international food
                                        standards to protect health of consumer and  to facilitate international food trade
 

ECOSYSTEM TOXICOLOGY

Single or multiple species; Microcosm; Mesocosm; Field; Region

• SELECT PHYSIOLOGICAL & BEHAVIORAL PARAMETERS WHICH PREDICT ECOLOGICALLY-SIGNIFICANT RESPONSES

• EXPERIMENTALLY MEASURE THE SUBLETHAL RESPONSES

• RECOGNIZE CRITICAL EFFECTS AT DIFFERENT LIFE STAGES

• EXTRAPOLATE SIGNIFICANT RESPONSES TO THE ECOSYSTEM

l. BASED ON CHANGES IN ECOSYSTEM FUNCTIONING
2. BASED ON CHANGES IN COMMUNITY STRUCTURE
3. BOTH

BENTHIC MACROINVERTEBRATES, ALGAE, FISH, etc.
No. of higher taxa, families, genera, species
No. of species & species diversity (age group)
 
 

ORGANOZATIONS RESPONSIBLE FOR REGULATION
USA     National Toxicology Program;     EPA;    FDA: Panel on Carcinogenesis;     FIFRA: pesticides;
    Secretary’s Commission on Pesticides; Panel on Carcinogenesis;    Past Presidents of SOT Task Force;
    NAS;    Toxic Substances Control Act,  ASTDR, NTP
Europe    OECD: Codus Alimentaius
UNO     FAO/WHO: Expert Committees on  Food Additives and Pesticide Residues
               International Agency of Research in Cancer
 

• STATISTICAL MODELS FOR CANCER RISK ASSESSMENT

• DISTRIBUTION MODELS. These assume that every member of a population is insensitive to a critical low dose, which varies for different individuals.
• LOG-PROBIT MODELS. The distribution of log-10 dose-probit response is normally distributed. Extrapolations are to 1/10,000%.
• MANTEL-BRYAN MODEL. This uses a fixed slope of 1 starting from the upper confidence limit of the observed proportiona of animals with tumors at the exposure level. The true dose-response curve lies somewhere below the extrapolation line passing through the upper confidence limit with a slope of 1, therby producing a higher proportion of tumors than that which actually occur at lower levels.
• WEIBULL MODEL. This uses weighted least squares regression endowed by Food Safety Council

STOCHASTIC MODELS

• One-Hit (linear) model. This assumes the DNA exposure to a single molecule of the carcinogen. The dosage-mortality is on arithematic scale.
• Mechanistic models:  A tumor originates from a single cell
• Gamma-multi-hit model. This is based on the requirement of more than one hit at the cellular level, cancer being a multi-stage process that can be approximated by a series of multiplicative linear functions.
• Armittage-Doll model. This assumes that the induction of cancer follows random biological events, the time rate of each being in linear proportion to the dose rate.

Pharmacokinetic models.