Cumulative Exposure
Summing RF Dose Across Co-Located Sources
RF safety standards recognize that real environments rarely contain a single emitter. A worker on a communications tower may stand within the near field of a cellular panel, a microwave backhaul dish, and a paging transmitter simultaneously, each operating at a different frequency and duty cycle. Cumulative exposure captures the combined effect by normalizing every source against its own frequency-dependent limit, then adding the fractions. The result, often called the total exposure ratio or exposure quotient, is dimensionless: a value of 0.45 means the worker is at 45 percent of the maximum permissible exposure when all active sources are counted together.
The reason fields cannot simply be added in watts is that the Maximum Permissible Exposure (MPE) and SAR limits are not flat across frequency. Below roughly 30 MHz and above 6 GHz, the limits relax or tighten according to how energy couples into and heats tissue. Whole-body resonance, which falls near 30 to 40 MHz for a grounded adult and near 65 to 80 MHz for an isolated one, drives the most restrictive power-density plateau, broadly the 30 to 300 MHz band, while at millimeter-wave frequencies the absorption is shallow and surface power density becomes the controlling metric. Normalizing each source to its applicable limit before summation is what makes the comparison physically meaningful.
Time is the second axis. Standards permit brief excursions above the instantaneous limit as long as the energy integrated over the averaging window stays within the time-averaged budget. A transmitter running a 20 percent duty cycle delivers only one fifth of its peak-power dose over a 6-minute window, so duty cycle and source-on fraction scale the cumulative ratio directly. This is why a pulsed radar and a continuous-wave link of equal peak power can produce very different cumulative exposures at the same standoff distance.
Computing the Total Exposure Ratio
TER = ∑i ( Si / Slim,i ) ≤ 1.0
SAR basis (local and whole-body):
TER = ∑i ( SARi / SARlim,i ) ≤ 1.0
Time-averaged source contribution:
Savg = (1 / T) × ∫0T S(t) dt ≈ D × Speak
Where Si = time-averaged power density of source i (W/m²), Slim,i = limit at that frequency, SAR in W/kg, T = averaging window (360 s typical), D = duty cycle. Example: three sources at ratios 0.18, 0.22, and 0.05 give TER ≈ 0.45, or 45% of the limit.
Limits and Averaging Windows by Standard
| Standard | Tier | Whole-Body SAR Limit | Local Peak SAR | Averaging Window (WB / local) | Summation Rule |
|---|---|---|---|---|---|
| FCC OET 65 | Occupational | 0.4 W/kg | 8 W/kg (1 g) | 6 min / 6 min | ∑ ratios ≤ 1.0 |
| FCC OET 65 | General public | 0.08 W/kg | 1.6 W/kg (1 g) | 30 min / 30 min | ∑ ratios ≤ 1.0 |
| ICNIRP 2020 | Occupational | 0.4 W/kg | 10 W/kg (10 g) | 30 min / 6 min | ∑ ratios ≤ 1.0 |
| ICNIRP 2020 | General public | 0.08 W/kg | 2 W/kg (10 g) | 30 min / 6 min | ∑ ratios ≤ 1.0 |
| IEEE C95.1-2019 | Unrestricted | 0.08 W/kg | 2 W/kg (10 g) | 30 min / 6 min | ∑ ratios ≤ 1.0 |
Frequently Asked Questions
How do you combine RF exposure from multiple transmitters on the same site?
Sum the fractional contributions, not the raw fields. Each source contributes a ratio of its time-averaged power density (or SAR) to the limit at its own frequency, and the cumulative total must stay at or below 1.0. A 700 MHz panel, a 6 GHz Wi-Fi node, and a 28 GHz 5G array each reference a different MPE, so contributions are normalized before they are added. FCC OET 65 and IEC 62232 both mandate this ratio-summation for co-located, multi-band sites.
What averaging time applies to cumulative RF exposure?
It depends on standard, metric, and tier. ICNIRP 2020 and IEEE C95.1-2019 average whole-body SAR over 30 minutes but local SAR and power density over 6 minutes (360 s); the older FCC rules used a single 6-minute window for occupational and 30 minutes for general-public exposure. Brief peaks above the limit are allowed as long as energy integrated over the full window stays within the time-averaged limit, so duty cycle scales the cumulative result directly.
What is the difference between cumulative exposure and peak SAR?
Peak spatial SAR is an instantaneous local maximum, the highest 1 g or 10 g absorption anywhere in the body, checked against a fixed limit such as 1.6 W/kg (FCC, 1 g) or 2.0 W/kg (ICNIRP, 10 g). Cumulative exposure is time-integrated and source-summed: it asks whether total dose over the averaging window stays within budget. A device can pass peak SAR yet still raise cumulative exposure if it runs a high duty cycle beside other emitters.
Does duty cycle reduce cumulative exposure?
Yes, directly and proportionally. Because the limit is time-averaged over the window (typically 360 s), a source transmitting only part of the time contributes its peak power density multiplied by its duty cycle. A radar at 10 percent duty cycle contributes one tenth of its peak-power dose. This is why TDD and pulsed systems often clear cumulative budgets that an equivalent peak-power continuous-wave link would exceed.